They've been pretty clear that it's not Shep. The last I heard (not sure if it's a rumor or what) but somewhere in the range of a thousand years in the future. A Krogan has definitely been confirmed. The other races? Wait and see.
As one of the few people that didn't completely hate ME3's ending (5/10 at best IMHO, but certainly not the abysmal 0 some folks paint it as), I'm actually curious to see how they deal with the story moving forward. I definitely like the idea of ancient advanced cultures becoming myths and legends, but if that's the case, how did N7 survive? Has that become a legendary symbol? I suppose only time will tell. I try not to judge too harshly before the game comes out.
My hope is that this new entry in the series will return to the much more open-world feeling that the first Mass Effect had. Or open-galaxy anyway. I definitely like the idea of space cowboy. Not enough sci-fi games have that kind of feeling to them.
It looks like the planet textures have been adjusted greatly in order to ease the Mako's handling, which is nice, I just hope they actually make it purposeful instead of driving around looking for... minerals... I'm no geologist.
I thought a prequel would have made a much more interesting game, but, with these entertainment companies and their constant rehashing of old series, I suppose it can't be ruled out.