I just hate Roy, a lot. His only popularity is through Smash. The same can be said for all of the other FE characters in Smash (Marth outdated, Ike overrated, Leif activated!), but Roy takes the cake. A whiny kid from a Japan-only game that is mid-tier for FE games would never have been this popular if it weren't for Melee. I severely dislike how arguments are made for Roy's (and other characters') inclusion is only for seniority. Franchises like FE and Pokemon have many good candidates and rotating characters in their games that should be added but are left out because of work put into veterans. Jigglypuff was almost cut 3 times but kept because she was a part of the original 12. Pokemon has 6 reps and 7 gens and only reps 3. Now people want Roy, mainly because he's a veteran. FE has 6 reps that don't represent popular games within the series like Blazing Blade or the Jugdral games. Not saying that Roy can't have an appeal, but there are so many better options.
I just ok with Lucina, don't really care that much because her inclusion doesn't hurt anybody. She reps an important game and is extremely easy to implement if kept as a clone (another reason I argue against Roy, work would have to be put in to stick to new moveset/declone him more.) As many have pointed out, she was forced into FE Warriors by Nintendo and IS, so she has to still be considered important. Some even speculate that she's being made the new face of the franchise and that Marth is slowly becoming a relic of the past. I'd rather her be an alt though, no offense, I just think a lot of lords as Marth alts would work really well since most share similar fighting styles.
I say just cut Corrin. Corrin is quickly losing popularity/relevance as shown in the 2017-2018 CYL polls, he moves from 5th to 9th. Fates is dying out and was only a success commercially, Corrin is considered the worst lord of the franchise. A moveset will be lost, but it's for the better. Unfortunately, we might actually get more Fates content since Smash 5 is being developed right after Fates' international release. I guess we'll have to see if we can finally be free of Fates promotion.
Cut Roy and revert Lucina to what she was supposed to be: an alt. However, I think
this could be a solution for the FE character problem. Sure, Roy would lose his decloned moveset and unique attributes, but he'd live on as a Marth alt with Lucina and the other rapier-lords. This would also allow for more FE lords to be repped while allowing focus to be put onto more important things.