I'm big on immersion in role playing games. It's not much of a 'Role' to be 'Playing' without it. So one thing I missed from ME2 and ME3 was real time elevators and embarkation/disembarkation. Yes you got to where you where going faster in the later games but it lacked that immersion quality and I'd love to see it restored to ME4. I also never liked the SR2's Engine room. It felt too small, too nuclear reactor-like rather than ME1's full on sci-fi glowing pulses and spinning bits.
Ditch the crappy endings in ME3 all together and center this new Mass Effect on something else please for the love of god. Also bring back advanced, and I mean advanced Customizations to keep the games RPG's like they're supposed to be, and make sure the new endings aren't like ME3's at all.
After playing the original endings in ME3 then hearing about ME4 in the works, it's always made me wonder how on earth they're going to reconcile a fourth game with those endings as canon? We're either all biological/synthetic hybrids, Shepard committed genocide or the Reapers mysteriously disappeared, leaving the entire galaxy at the mercy of the Leviathan race to once more dominate the galaxy with mind control.
Then there's the issue that due to the fact that since only the Prothean's so far have managed a working prototype of a Mass Relay and all the Reaper-built ones have exploded, the entire C-space race's fleets are stranded as refugees on an earth that has suffered near total infrastructure destruction, and even assuming there's enough food to feed them all, half of the races can't even eat human foods because it's poison to them. I mean, the ME3 ending has more holes than a slice of Swiss cheese. If they've any sense they'll retcon the whole damn ending.
There's another thing I wanted to say. I know Bioware aren't shy when it comes to plagiarising Warhammer 40K but some of those screenshots of the 'dark world' look suspiciously Necron in their design. How about some originality?