On continuing a universe with a new protagonist:
It's been done before in other games. In each Fable game you play a different protagonist, and Halo did it with ODST and Reach where you played as the Rookie, and Noble Six respectively.
The Crysis series did it too, with Crysis Warhead you play as Psycho, instead of Nomad and in Crysis 2 you have Alcatraz. And specifically in the Star Wars games there is KotOR 1 and 2 where in the second game you play the Exile instead of Revan and KotOR 2 happens only 5 years after the end of the first game.
And also the Dragon Age series, with the Warden then the Hawke family.
So Mass Effect's universe can continue without Shep. But the challenge would be making a story compelling enough to live up to the first three games. Which is completely possible because so far ME is full of plot holes and is essentially an idiot plot.
(Really in ME2 even if you take Legion to meet the council they immediately dismiss him as a trophy. You have essentially what is the only non-hostile ambassador of the Geth beyond the Vale and they don't take 30 seconds to consider speaking to Legion directly. Not only that but you meet the council after getting the video of the collectors actually abducting a colony and they dismiss it saying it's not in council space so we don't have to worry about. I'm sorry but that's an idiot plot because it fails logically on so many levels.)
Addendum: Tali's face.
I don't think they should show her face and probably shouldn't show the face of any quarian. Why? Because the moment they do, rule 34 will distort her face worst than anyone should care to imagine.