[quote=@vancexentan]Hey everyone I've been considered making something with the titicular darling in the franxx series that is still airing. However I'm not sure exactly where I'd start with it. [/quote]
The biggest centerpiece of a good fandom-based roleplay is what your design philosophy is and what you believe is the spirit of the series you want to interpret. 

Once you get past what you feel you think the spirit of Darling in the Franxx is, you have to figure out where you want to take it and if you have the kind of player pool that would be committed to a long term collaborative story that is dependent on two writers always being present and dedicated to the project. Look at how Fate/Stay Night and Soul Eater oriented roleplays function and why some have failed and others have succeeded. You may even be able to gain some insight by looking at Naruto RPs since they are dependent on a three-man squad who are interconnected to one another. Also, if you want to change Franxx’s setting directly you will be pushed back on people who want a pure Franxx experience. The whole concept of duality, the vivid and unique world design, and the concept of these synthetic thermal energy hunting monstrosities are all what sets Franxx apart from its contemporaries like Fafner and Evangelion. 

Do you think making a static city (and explaining how it can exist) is an important risk you want to take with your potential playerbase for a currently trending anime?

[quote=@vancexentan] I'd flat out get rid of the gender exclusivity as in a roleplay we may not always get a set of people that are evenly male, and female.[/quote]
This is a bad design idea because it deviates too much from the theme of duality, male/female syntheses, and so forth. I understand not wanting to setup “gender ratios”, but once you deviate too far from the source material you lose what made the setting special in the first place. Be [i]very[/i] careful if you are making drastic changes.

[quote=@vancexentan]Really what I need feedback for is how I should go about making a 'plot' for something like this, and if I should relegate the monsters to a Co-GM, or allow the players to fight their own monster but that runs into the problem of when there is a need for one BIG monster.[/quote]
As a GM of one ongoing post-apocalyptic mecha roleplay, I would say that you should never [i]ever [b]ever[/b][/i] let the players control the threat unless you trust them wholly as creative partners and they are absolutely on the same page as you. I’ve never been in one of your roleplays, but as far as I can tell you make things that are very rarely invite only and super selective, so this would be a thing I would avoid. GMs should control the threats and create the suspense and action for the players to build off of.