If it helps, Gwen, I reported all those extra posts, heh. I don't know if you did too. Thankfully the site stopped borking long enough for it.
Oy, what a terrible day. D:
Anyway, more than extra costumes, I'd honestly love some unique echoes meant to emulate old movesets. I'm not sure if Echo is applicable either way, but one I'd love is Black Shadow using Ganondorf's TP moveset. This fits his dark magic factor well, and he's also a very strong built character who is more physical than usual. I also feel this fits better than Captain Falcon echo-wise due to both the dark magic and Ganondorf literally representing the spirit. Blood Falcon was his helper, fittingly. Though I expected the Blood Falcon helper regardless of who represented the Black Shadow spirit. Before I knew of the spirit, I always felt Black Shadow's physique heavily matched TP Ganondorf's too. So it felt like the perfect fit to me.
Otherwise, you have TP/OOT Link, Melee Mario(could be done by using an old artstyle or the earlier Metal Mario design, who has a hardline color instead of shades like the Metal Box gives. Not that great, but I'd love to see him as a character proper again after Smash 64. He also is actually different personality-wise now in Mario Kart, being practically a different person), Pit(he changed a lot of abilities into 4 from Brawl, and that's not just Gliding capability, which should be there. Though I'd have loved Dark Pit to have his Brawl moveset, but that didn't happen), Smash 64 DK(he has a lot of different moves. Wouldn't work for Giant DK since he can't really be told apart beyond size. He doesn't have the unique coloring like Metal Mario legitimately does, and he barely has enough to be worth considering for an Echo, imo), Melee Roy(due to a lot of animation differences and new hitboxes. Lucina definitely does not emulate this. Though maybe Chrom comes fairly close beyond the Up B? But then again, no sweetspot, so still very different), and Melee/Brawl Bowser(who was playable for a short time with the Giga Bowser Final Smash in 4. I don't know if Giga Bowser, the boss, still emulates Bowser's older playstyle, though). I can't think of other good examples of old playstyles that changed massively over time though. Luigi maybe, but he feels only slightly different once Melee hit. Pikachu lost his sweet up air for a while after 64, but I don't think it played all that differently?