things that I've run across through Mass Effect 2 that seem to good for the developers to leave hanging.
Preying Mouth - Seems oddly like that random planet from ME1 that suffered the Giant MAG Round that ripped a canyon through it which turned out to be *SPOILER* a battleground where a derelict reaper is located. I'm thinking will find out something about this.
Just a thought- do Tali's loyalty mission, I can only assume that the judgement of whether or not the quarians go to war with the geth will effect the gameplay of ME3. Something so trivial is just the thing to try to sneak past many gamers so that they smack themselves in the face not expecting the repercussions.
Extinct Races - I'm guessing that some of the earlier extinct races killed by reapers might make some sort of debut after raping the collectors. the mentioning of them (all the races) was drastically increased between ME1 and ME2 which makes since but I think theyll be greater explanation of their races.
Element Zero - said this once I'll say it again, it seems to have a heavy connection with planets that possess(ed) life. Think the developers will try to pull one on us
More Homeworld type venues - this is a given but the turian and hanar homeworlds will probably be explored.
Earth - I'm still debating whether or not earth will be brought in to play a larger role, but theyre pros and cons.