Corrin should 100% return for the next game. His moveset is entirely unique due to him being a Manakete/Dragon character and Fates did very well in terms of popularity and sales. I'd be absolutely heartbroken and disappointed if he doesn't come back simply because people were mad that 'we got another swordfighter', 'we have too many Fire Emblem reps now', 'he's too OP', etc...
If any Fire Emblem character gets cut, for the love of God let it be Roy or Lucina. In fact, just make Lucina an Alt for Marth and make some more changes to Roy to help him stand out as his own character. We'll probably be getting another rep either at release or via DLC depending on when the new Fire Emblem for Switch comes out, so reducing Lucina to an Alt would keep her in and make room for another rep so that people won't incessantly complain about 'too many Fire Emblem characters'.
Speaking of, that complaint is so nonsensical and annoying. Fire Emblem rightfully deserves these reps, it was a series that initially was essentially unknown to the West and Smash introduced us to it in Melee, which boosted it to the point we started to see releases in the west. Then fast forward a bit and things weren't looking good, the series was on the brink of retirement, then Awakening happened and skyrocketed the series into relevancy and popularity and since then has done remarkably well and is becoming one of Nintendo's well known IPs.
Also with a game that continually expands and introduces new characters very, very often like Pokemon and Fire Emblem, it's to be expected to see new reps for those series in each Smash game.