Good, well thought-out co-op mode would be awesome. That and making it a bit longer (or charing less for it. Either/or, really).
Add a few more "moves"... Maybe some limited climbing (it kind of annoyed me at times how some things were obviously climable, like the scaffolding, but you had to run/jump around it because the game said so).
Actually add multiple routes through some (not all) levels, so it doesn't feel like you're being shepherded through the level the whole time.
More interesting achievements.
Semi-freeroam levels (but not the entire game. The structuredness of the first was part of what gave it its character, but a few larger levels with a lot of back and forth across them would be nice once or twice, and they really let you extend gametime for limited extra effort).
Other people in the timetrials (e.g. adding ghosts in the 1st one. Maybe having a few freeroam timetrial levels with multiple non-ghosts and a tiny bit of combat to make things challenging).
If I were to add MP combat, it wouldn't be lethal, simply a method of slowing someone down or preventing them from following you, or perhaps knocking someone off of a building.
The ability to grab onto a building if you just walk off of it. I mean, seriously, why couldn't Faith do that in the first one? Also, see relevance to MP combat mentioned above.
A good, easy-to-use map editor with the ability to publish maps online, with a semi-decent way of having the public vote for them to keep them relevant. Also allow external hosting and modding for greater customisation. (i.e. don't limit it to just mods that can be aquired from whatever in-game method you use for acquisition).
"Better", more involving story. There are hundreds of good authors out there who can write stories better than most games. Why not get one of them to write something good, and then base the game's story around it, rather than coming up with an idea and getting people to write the story?
Faith not instantly being proficient with every gun you throw her way. She's not Gordon Freeman. I don't expect her to be a crack shot with a sniper rifle, or a crossbow, or the random SMG that the Police Force carry. Heck, I don't even expect her to be all that accurate with a pistol. It's harder than it looks!
No PhysX. (I don't want to have to play it on my laptop to make it look shiny
A similar graphical style, but with better texture quality and bump mapping.
Interesting mini-games. They don't have to make sense, but would be easy to add multiplayer to. A bit like what you had in Pokemon Stadium on the N64 - random little games that don't take too much time but add lots of fun moments with friends.
(The one I have pictured in my mind sounds crazy but would probably be fun to do - where the floor is separated into different moving pathways, a bit like you get in airports only without the barriers, and you have to run against them trying to make headway while dodging objects moving towards you, switching isles and pushing away opponents to get into the best lane. Attacking an opponent slows you down somewhat, meaning that you "do your best running" when not distracted. Add in a wide variety of interesting obstacles, perhaps the odd NPC and you have a weird mini-game that would probably take five minutes to play through at most, but could have an interesting multiplayer dynamic).
A police Vs. runner MP mode. Potentially with more than 1v1. Ideally if it's more than 1v1 having multiple routes would be good (another use of those few "freeroam" maps that I mentioned before). Hidden "shortcuts" everywhere, of course, to make it more than just about the running - also about spotting the best route as well as making the best lines.
There are probably a few more things that I'd put in, but those are the ones that come to mind, in no particular order.
Ultimately, I doubt much of that will make it into Mirror's Edge 2, if it ever gets made, but I'd love to see even a bit of it in there.Edited by Korlus - 3/2/11 at 5:55pm