Gameplay wise I think all past games have been largely great when judged on their own merits, the main things that can be improved is things like what Valdez as touched on and also engine issues. ET:QW had some great features like friendlists, clan formations and so on, expand on this further with being able to challenge other clans in game, have chat options and other tools like that, things to make it really easy to play with fiends and to join in games, not at the expense of a decent server browser though, again ET:QW had a really good browser.
Next thing is the in game options, why did we have to spend ages getting the perfect .cfg before? ET:QW improved on the older games by having loads of options in the menu to configure the game, the crosshair and HUD options were immense, you could set the individual alpha on pretty much everything and change colours of certain other things, through the menu. More stuff like this would be great, as to not overwhelm the average player tuck them away in a advanced settings sub menu, perhaps even a total HUD customisation menu, move, remove, colour, alpha and scale of hud elements on some HUD editor screen, that would be all kinds of goodness.
Video recording would be another thing I’d try to really improve on, having your players upload videos and share them is great, from silly little derp moments to offer a little chuckle to frag movies showing off how the game can be played at a high level, anything to get that sort of thing happening easier than it was in past games would be very welcome.
Last one can think of is just a general thing, make sure the game runs smoothly, I’m pretty sure ET:QW, Wolf09 (I know SD didn’t make it) and Brink would have faired better if the engine used for them worked better, even with mods that made the first two games mentioned more suitable for competition the smoothness of the game was very hit and miss and was frustrating at times due to the inconstancy in fps, ping and hit registration, wasn’t great to compete in that situation. The last one especially, was disheartening to be having an epic battle against a cyclops as a solider with a rocket launcher, get him down to low health, move in for the final shot, shoot, hit, no damage, then you die from his shot. In pub it wasn’t as bad as there was a lot more happening but still frustrating.