I'll obviously start. Hopefully i won't be the only one who bothers with it, although it is probable.
. Tali & Garrus are two npc's that are absolutely required in the third part of the series. Not only are they major part of the story, made sacrifices to stay with Shepard, but also they are also great characters.
From the second ME i guess Legion, and the DLC ones of Kasumi and Zaeed are the most interesting ones, that could benefit you the most. Perhaps Grunt for beign straight-forward, and a pure-krogan with pure-krogan quests.
Also Jack.
From non-joinable NPC's.. I guess Aria, Chambers, the reporter making [url="http://www.youtube.com/watch?v=DFM-PvXWcqo#"]disingenuous assertions[/url] aswell as [url="http://www.youtube.com/watch?v=6RMFwzoxCUM&feature=related"]snide insinuations[/url] is a must
last, but not least..
"Ah yes, Reapers" council member, and the option to rub it in his face. He doesn't have to be alive, really
. The two humans from ME1. Kaidan and Williams, shouldn't re-appear as joinable NPC's.
Miranda shouldn't appear as joinable NPC.
Same as Liara, but in this case story-wise.
Mordin shouldn't re-appear as joinable NPC if you gave him the quest to work on genophage.
I'd rather not have a hanar or one of the wall-mart aliens ( " Emitionlessly ; welcome to wal-mart" )
Non joinable, i don't care, as long as you can kill most of them should they annoy you.
. ME 2 combat system was very, very good. A bit too focused on cover however. So while i'd like a full import of ME2 combat system, i'd also like much, much more agressive and larger number of enemies.
. ME1 combat system.
. I'm not sure about ammunition. I liked the overheating of ME1, but i also liked having to kill with every shot (insanity ME2, infiltrator)
. ME2 "Insanity" should be "normal" difficulty in the third part.
.10 levels of dos mario for every lock you want to pick + annoying music.
.Cheesy method of getting rid of the reapers.
. I bet a penny that Shepard will die at the end of ME3. No way around it.
. ME2 had very nice renegade interrupts. The more the merrier.
.A lot, all of them optional.
.None.
Regards!