For me: Kick the singleplayermode (Noone bought UT/Q3 for singleplayer and look how well it sold), focus on multiplayer. Include as many options as possible (adjustable time, special items on/off), mutators (instagib, low grav), many gametypes (CTF, Rocket Arena, Midair, LMS, Bombing Run (duh^^) etc.).
Maps with DIFFERENT SIZES that dont have to be symmetrical -.- (at least not if it isnt CTF). That bullshit about "wow, our maps are so big" sucks. It depends on how many players are on it. 1on1 is so different from (Team) Deathmatch or CTF. Look at UT (not onslaught)/Quake.
But the most important thing is BALANCE (Weaponbalance (poor in Demo), Item respawntime (!) the most important thing in UT 1on1 is to time the items like big shields etc. In the MPH-demo all items appeared too fast when picked up and you could get too much health too fast.
Maybe implement dodging (doubletap the strafebutton like with tha ballboost to do a quick hop to the side and speed up the gameplay.) Oh and make using the Morphball reduce your defence to half because it was so annoying already in MP2 when someone just rolls around and there is no way to stop him with even well aimed shots because they did no damage -.-
Phew. So my biggest fear is that Nintendo does a half-good Singleplayer Adventure and a Half-good Multiplayer... We already have Good Singleplayer Adventures with Samus... (oh and take notes from Tony Hawk's Online Features!!!!)