I had no idea about the Arkaneia stage. That's very interesting.
For likelihood for a new rep, it all depends how much you gauge fan response in relation to Sakurai's choice. Sakurai doesn't necessarily take fan popularity into consideration (at least not with other factors). If you think fan popularity affects Sakurai (to such a degree that it's more important than other factors), then I would say the likely choices would be Chrom, then Roy, then Lucina. Sakurai has proven otherwise with other series. I personally don't think fan popularity affects him (unless he says it does) so I think it's Chrom and the Lucina and Roy, with Lucina edging out Roy but both very much behind Chrom. In terms of want, it's Lucina, Lyn, Ephraim, and Anna in that order.