Timeline: http://axleonex.deviantart.com/art/Omegaverse-Complete-Timeline-505752402
Lore Template:http://axleonex.deviantart.com/art/Omegaverse-Complete-Guide-505969268

I've created this crossover universe that I'd really like to roleplay with some people (see links up top).

I have a story outline for how I'd like this story to play out. STORY SPOILER HERE: 

[hider=spoiler]

2537: the Covenant attack and destroy the UNSC colony of New Constantinople. The reason this is important is because I plan on having a mini plot revolve around this. Basically, the quarian fleet (From Mass Effect) was around that planet at the time, and Earth decided to send their entire fleet to save the quarians from extinction from the Covenant. The result is that the quarians would be granted refugee and live on the planet Earth in Antarctica, where they would create a city called Sanctuary.

Another thing that would occur this year is that the ORION project data from Halo (The thing that made spartan soldiers like master chief) would be leaked into the public. It would be discovered that all helghans have the perfect genes to make spartans, and the Helghast empire (From Killzone) would begin implementing the spartan program into all of their soldiers.

Another thing leaked from the Martian alliance is that the Martian alliance was ALLOWING outer colonies to be wiped out by the Covenant. They allowed it to happen because they wanted the colonies that were seceding to be destroyed. This leads to massive disapproval against ht martian government, and eventually lead the people of Mars to overthrow the government. (But that'll happen in like twenty years)

2546: The Skylian blitz from Mass Effect. Saving the citizens that were captured from the Skylian blitz. And attack Torfan, the pirate planet responsible for the Skylian Blitz. These arcs are more action then conspiracy. Mercenaries and soldiers from all across the galaxy will participate in this, making the event very diverse. 

2549: Promethean becon discovered on the planet of Helghast. Events in Mass effect, Dead Space, and Crysis occur on this year.

2550: Events in Halo take place.

2251: The Reapers from Mass Effect attack.

[/hider]

Something important to know is that the story will follow through to course several years. The first year starting at 2537, the missions around this time is to give a basic understanding of the universe we'll be playing with. 2546 will have a few missions meant to introduce your REAL MAIN characters (unless you plan on using a character that can stay in good shape after a lot of years). While 2549 the main stories from the original video games will take place.

Make up your own character if you'd like, but here are some "Main" characters you can reserve for the second arc:

[hider=Canon Characters]

Master Chief is a Helghan from Killzone
[img]http://fc04.deviantart.net/fs70/i/2013/016/d/9/beast_marine_by_domls-d5rog1t.jpg[/img]
Samas Aran, the bounty hunter from the Terminus Systems.
[img]http://th00.deviantart.net/fs70/PRE/i/2012/324/7/c/n7_samus_aran_by_kaj18-d5ln8q7.jpg[/img]
The Shepard twins work for the Systems Alliance
[img]http://th02.deviantart.net/fs71/PRE/f/2014/068/c/6/_twin_fates__cover_art_by_dblack930-d79hwoj.png[/img]
Cortana is one of the many AI that work for the UNSC
[img]http://fc04.deviantart.net/fs70/i/2013/121/b/e/cortana_colored_by_oomnine-d5vwvts.jpg[/img]
And possibly the Vault Hunters that reside in the Eden Wastelands
[img]http://fc09.deviantart.net/fs70/f/2014/108/0/7/the_vault_hunters_of_borderlands_1_and_2_by_glench-d6ik4wu.png[/img]
And possibly some ancient reploids from Neo-Arcania (Formally known as Taiwan)
[img]http://fc04.deviantart.net/fs71/i/2012/122/a/b/megaman_vs__zero_by_kofab-d4ydf6a.jpg[/img]

[/hider]

It's alright if you haven't played all the games on this list (because I sure haven't). But you probably do need to have a good understanding of the Halo and Mass Effect universe, the rest can be understood by the lore guide that I created.