I am pretty confident it will be Wrex's brother. I don't think Grunt will have much of a role in ME3. I don't think any of the ME 2 squad will be very prominant in ME 3. This game will likely be about Liara, Garrus, Shepard, and the virmire survivor. Hell I don't even think Garrus will be all that important since he can die in ME 2.