well.. It IS shephard but whatever:
IF your scenario is correct, then i would say this:
Anders is a no-go. He have laid off of combat in ME2 and he's occupied. If Earth get invaded by Reapers, he would be the last guy i would expect to be on the front lines.
Joker, according to your prediction, would be the new leader appointed by Cerberus. Problem here is that Cerberus IS an enemy in ME3, atleast that's what i gather from the screenshots + info. Could be wrong but heck, i thought he was waaaay too injured to be fighting.
Wrex is Krogan... i don't think we will ever have a main character who is not a Human for Mass Effect, considering the major themes throughout ME2 of racism (i guess that's what it was) towards humans by almost any other race. It was the point that Shepard, despite his renown was being hated by some simply because he was a human. Hence useing a figure whom we would relate to (since we are humans, last time i checked) put ourselves in the shoes of the subject of racism. It's kinda one of the deep themes i thought of while playing through the game... Back on track:
Wrex is not a possibilty either, he's krogan, he has an army at his disposal which i can't seem to get to work with the game. Combat experience is a yes but also a HUUUGE yes. He would be an unstoppable killing machine if a player were to be him. He is a krogan, and a damn strong krogan too so no. Too imba
Liara is also a way too strong and succesful person for it to be fair for the player. too imba. With all the military and money at her disposal, it would ruin some parts of the game. Also, her entire arc is sort of a side-character... As you said, her desire is to finish what her lost love started, allready there don't we know 100% of what she means. bioware would have to make up a loooooooooooooot of story to cover up that part i think...
Williams / Kaidan are the most qualified one here. They are both decently trained, as was Shepard, They have met the reapers first hand which is also great. They dislike Cerberus, the enemy, which is also a goot point. They are not too "strong" or anything so they would make the perfect fit for Shepards role if you ask me. Not too might and not too weak to be the main character.
At the end of the day we still know that it IS shepard who we will play as, was interresting though... would've been more interresting had you posted this the second we got the announcement about ME3
