Let's not forget the (optional) revives of Chidori and Shinjiro.
Way I see it, Liz is gonna try to save the MC by becoming the Great Seal herself instead of him.
Seeing Liz popularity i hardly think that would be the case. people say that the message of P3 would be lost if Mc is revived but i think the message of reviving him would be stronger: "Even if few people wish it they have the strength to change humanity negative feelings", or something like that.
I'm pretty sure Igor will be gone as well, but I figured Marge would replace him.
As for the Igor VA thing, they went out of their way to use his voice samples straight from the game in the anime, severely limiting his lines, but as a tribute and out of respect for the original VA. Had they intended to replace the voice actor, they probably would have done it there.
Lelouch WOULD make a pretty good Igor, especially with the black ranger voicing him.
But seriously, Igor is much flatter than Margaret, personality wise. Although, it would feel kinda awkward/angry to walk into a limo with two hot chicks with smiles of shame and a dude that looks like he just got his basketballs polished.
I'm currently replaying the very first Persona game and I gotta say: Igor is amazing. He can't be gone.
By the way, about Guilty Gear, they dropped the character Zato-1 in terms only. In terms of storyline, he died and Eddie took over so they could use another VA, but for all effects, the characters are exactly the same. So much so that someone who plays Guilty Gear without knowing the storyline would have no idea that Eddie isn't exactly Zato. Basically, they just found a creative way to circunvent the problem, but if that wasn't an option, you can be absolutely sure that they wouldn't simply drop the character because its VA died. [Edit: got this last but wrong, but this is still my opinion.]