I was almost hoping that there wouldn't be any returning characters =\
Looks like Liara is here to stay, wouldn't be surprised if she was in every game from now until the franchise breathes its last ragged breath.
If next ME game is set like a century after ME3, Wrex, Grunt, Javik and Miranda may show up.
Wrex and Grunt because krogans live over millennium long.
Javik because Protheans most likely should live long too. As well Javik can only die in the worst ending, but that's pretty much game over as Reapers won.
Miranda because she can live twice as longer then average human.
However, I'm speculating. Nothing says yet will the next ME game will be set before, during or after ME3.