It's 1923, and mere weeks ago, a vicious, decade-long war over the region of Orre came to it's conclusion.
It'd been fought by the united military might of Johto and Kanto, against the ever-expanding interests of the Sinnoh region.
During the war, Orre- which, up until then, had been a series of roving valleys and prodigious mountains, rich with the valuable ores for which the region had been named- was rendered a barren desert wasteland as a result of the escalating violence of the conflict, and the Johto/Kanto alliance was forced to evacuate the territory following a crippling defeat in Phenac City.
Despite this, however, the dust in Orre still hasn't quite settled, as it's people- those who'd settled from all around the world, back when it consisted of winsome, idyllic countryside- have risen up in an attempt to force Sinnoh's invasion force back across the borders.

We are not, however, following the story of this new war.
Instead, our tale begins in the city of Goldenrod, as the people of the 'classic' regions settle back into civilian life, their "state of emergency" now considered totally forgotten.
But that is not to say there's peace in this bustling metropolitan area, not at all.

As the war had escalated in the latter-half of the conflict, the nation's officials had passed an act that resulted in the drafting of many law enforcers; This, in combination with the amount of families that- due to the conflict- found themselves without a "bread winner", precipitated a sudden bolstering in petty and organised crime in Johto's major cities; Crooks have found themselves quite suddenly at the top of the food-chain, and now those returning from the war are faced with a new battle... 
That between law and lawlessness. 

With no enforcement to stop them, already existing organizations, like the likes of Team Rocket, have expanded their brackets of control exponentially, and- through sleazy tactics and highly illicit but nonetheless mainly anonymous activities- have taken political control of Johto's capital, Goldenrod.
Their founder, Giovanni, now sits in the mayor's chair, and keeps his hands pristine and clean whilst his army of bandits and underlings perform his darker duties in his name.
This work has taken a turn for the dark and despicable, however: 
Having risen [i]above[/i] the minor crime of Pokémon theft, Team Rocket has now extended a campaign of fear over the people of the city, organizing violent terrorist strikes (often in the form of Pokémon-affiliated bombings) and mass Pokémon robberies from local storage centres (which have recently been disconnected from the PC systems, for valid concerns regarding the presence of hackers among Rocket's ranks.)
Try as they might, however, the newly returned police-force are powerless to help: The Mayor keeps his hands firmly clean, and claims to have abandoned his old comrades years ago. 
This, we know, is most likely a lie. 

The roleplay will follow the likes of those returning from the war, or those who lived through it, as they try to settle back into civilian life whilst combating (or [i]assisting[/i]) Team Rocket's unlawful agenda. 

So, without further adieu...

[I]How're Pokémon battles gonna work?[/I]

This is by far the trickiest part of the roleplay, because battling in a traditional collab would most likely take massive, train-stopping periods of time. 
So, for the sake of convenience, I'm going to need to ask potential trainers to either complete the posts in one sitting, or else take them to the likes of Skype or Titanpad.

As for the actual combat, this will be mostly down to your roleplaying capabilities: If you're the type who doesn't like to lose, you'd be better avoiding this roleplay all together, because it's inevitable.
Utilising individual stats would be very inconvenient, so the outcome of a battle will rely mainly on three factors: Size, type advantage and your capacity as a roleplayer. 
In terms of size, it's simple knowledge: If it's a Snorlax Vs. a Jiggypuff, you can bet your Pokédollars that the Snorlax is going to be hitting harder. 
Then there's type advantages, once again very basic: If a Growlithe uses a flamethrower on your Bulbasaur, it's gonna [I]hurt.[/I]
With these in mind, the true outcome is based on your roleplaying: Your battle strategy, and how fairly you play it out.
You're being trusted to make the fight reasonable by and between yourselves. 

However, as I can't fully rely on the honour system, this is where the likes of Skype and Titanpad come in. 
The idea is that, whenever a battle is formally entered into ("I challenge you!"), then it should be taken to one of these programs and written as a collaborative piece, with myself or another GM monitoring to make sure everything remains fair.
It's simple from there, really: You get a turn in which you can use an attack (Healing items have yet to be invented, meaning it's the Pokémon centre for your defeated pokémon), and then it's your opponents turn, in which they may respond and attack back. 
Your pokémon will be given a limited amount of dodges per battle as well (Let's say, three,) and each of these will make your Pokémon all the more exhausted (dodging takes energy, as it requires you to move your entire mass simultaneously and at a moment's notice) after which you're basically depending on the GM's decision on whether or not you can dodge further (every time you try to dodge after your allotted three, the spectator will decide if you manage it, although it's unlikely.)

Too long, didn't read: You're going to roleplay your battles instead of leaning on stats as a crutch, and it's basically dependent on you not being a military-grade douche nozzle. If you play fair, you've got a fair chance. 

[I]So, what Pokémon can I have?[/I]

Anything, more or less! Excluding the Pokémon from any region post-Sinnoh (because, as of 1923, they have yet to be discovered), you can pick anything you'd like!
However, your party is limited to one starter Pokémon, should you happen to pick one.
You can't have shinies, however, nor can you have legendary Pokémon, even if it's the sort of legendary in which there might be more than one.
"What if I want my pokémon to be individual, then?"
Give it a personality! Or a neckerchief. Neckerchiefs are neat.

[I]So, can my Pokémon learn TMs and stuff?[/I]

Sorry, this is the 20s- no TMs in sight, certainly not until the 60s.
Any move your Pokémon learns, it has to learn it naturally- and before level 50, preferably. 
By naturally, though, I mean along the lines of the roleplay: Obviously if a Pokémon can be taught a TM, then it can move the learn somehow! Rigorous training, in this case!
And that's right, fifty is your limit. Although you can get a Pokémon that evolves over level 50, level 50 is your limit for move learning. 
We aren't factoring levels into the roleplay, nor are we really taking into account the individual damage-count of each move, that's just the boundary for the sake of keeping everything tidy and fair. 

[I]So, why no Unova Pokémon?[/I]

Funny story, the post-Sinnoh regions haven't been discovered yet. Or rather, they haven't been populated and [I]explored[/I] yet, meaning we've no idea what Pokémon live there. 

[I]So, what's the plot? Collect gym badges?[/I]

Funnily, this is actually prior to gyms being established. The plot will follow a group of civilians and ex-soldiers whom, after the war, were forced to live within the confines of Team Rocket's Goldenrod, and either fighting against them, or submitting to the regime and helping them crush any resistance they might come across.
For the benefits of the roleplay, Goldenrod is a metropolis large enough to house political opposition and conflicts, as opposed to a tiny section of the game map. 

[I]How do we catch new Pokémon?[/I]

Ahh, here in lies the catch...
Team Rocket has made it dangerous to leave and re-enter the city's boundaries, meaning that whilst you [i]can[/i] catch new Pokémon, you'd need to be sneaky, and the chances are you wouldn't be able to go particularly far for them. Johto and Kanto Pokémon remain in their respective areas (as of HG and SS), and Hoenn and Sinnoh Pokémon need to be shipped across the ocean, which could put them at risk of being stolen by Team Rocket (and even if they aren't, Sinnoh pokémon are rather risque lately, the war having just ended.)

You will start with six Pokéballs, but you can only have a party of four Pokémon at a time (and will begin with none in storage), and they will be contained in your starting six, giving you two spares. You can buy more later, but never carry more than six at a time. 

[I]Only four?[/I]

That's right, only four Pokémon. As of the crime-rate rise, law enforcement has made is illegal to carry more than four Pokémon on you at a time- partially because it means you are less likely to abuse Pokémon, with each one makes up a quarter of your team, but mostly because it means Team Rocket members can't pack as much heat as they'd like (and if they do, they're arrested).
Another benefit, however, is that four pokéballs are easier to conceal, and thus are less likely to be stolen. 

[I]Wait, no levels? How does training work?[/I]

This is another aspect you're going to have to roleplay. Each victory will make your Pokémon a little more adept at battling, and as time goes on they'll become better at communicating and working with their trainer, to boot. It's up to you to show this improvement without overdoing it: Maybe start the roleplay with a relatively new, inexperienced Pokémon, and end it with a well trained partner to be proud of!

[I]Can I use evolution stones?[/I]

Yes you can! Evolution stones are still being sold in Kanto, as they haven't seen the same increase of crime as Johto has, being a mainly rural and farm-based community.
Because of this, the superstore that Goldenrod boasts is still supplied with these stones regularly: Although, they are very slowly becoming increasingly rare due to Team Rocket's insistence on getting their "cut". 

So, hopefully that covers the simpler questions... 
Here's hoping we see some activity!