I do think the best way to make all parties happy if for FE to get four slots this time.
Marth, Roy, and Ike are obvious. However, the fourth slot is contentious. Chrom/Lucina are the latest lords, but they don't really bring anything new to the table, and especially don't contrast themselves with the the previous three.
I still think that a fourth FE character is going to be some un-orthodox choice (like Pokemon Trainer or Zero Suit Samus). I think Sakurai if he pays close attention to the Western fanbase will notice how at one point Lyn was highly requested, then Micaiah was, then Caeda/Sheeda was, and now Lucina/Anna. If Sakurai is smart, his assessment will be that there is a significant contingent of Smash Bros. fans who want some sort of FE newcomer that is female, and maybe brings a new weapon type to the table. I think in that situation, it will probably be Anna that gets the closest look, as she is "evergreen", although she plays an unimportant role in every FE she appears in (her most important roles were in FE9/FE10 where she hosts the tutorial, and FE13 where she finally becomes playable).
I also think if Ike returns, he will have a design amalgration of his FE9/FE10 designs that makes him look somewhat similar to Chrom (much like Marth's re-design makes him look very similar to Lucina).
I also predict if we get Marth, Roy, Ike, and Anna, there will be an Event Match called "Red vs. Blue" in which Roy fights Ike due to FE13 having a DLC arc with the same name. In the co-op version, Anna will be Roy' s partner, while Marth will be Ike's.