Somewhat but with important differences.
In the case of Roy, he was really fortunate to have extremely good timing because as he was only considered
after Sakurai looked for more additions after getting all the base characters completed in Melee. He had perfect timing as his game was slated to be released around the time of Melee. More importantly, he was similar enough to Marth to be feasible as a last-minute clone character whilst distinct enough in the eyes of Sakurai to be interesting. If it weren't for the last two points and Sakurai loking for clones after getting every other character completed, Roy wouldn't have been considered.
In the case of Greninja, extremely good timing was critical. The most favourable Pokemon game in terms of timing was X and Y, which was released in the sweetspot of ~one year after the roster was finalized but ~one year before Smash 4 was released. In that regard the closest comparison in terms of timing for Smash Switch when it comes to FE would be Fire Emblem Echoes. Assuming the roster was finalized in 2016 like people speculate, Shadows of Valentia (similar to X and Y) would have been the FE game with the most favourable timing, being released ~one year after the roster was finalized but ~one year before Smash Switch is set to release.
I think the better comparison to a FE16 character would have been if they included a FE10 character in Brawl. This is because similar how FE10 was released in the same year Brawl was originally intended to, FE16 would be released in the same year Smash Swicth is set to. But we didn't get a FE10 character. We got Ike, and he was heavily derived from Path of Radiance which was released around the same time the Brawl roster was finalised somewhere around 2005 (with one obvious exception).
Heck,
Sakurai himself stated that had Awakening been released several months earlier or later, Robin might not have made it in, so timing is extremely important.
Honestly speaking, if a FE16 character is to be playable, I'd expect it to be more of a Roy situation (being a last-minute clone) than a Greninja (due to timing).
Pretty much, and we have also had newcomer characters whose only major, recent game appearance at the time was a remake (
due to FireRed and LeafGreen) so Shadows s of Valentia being a remake would not necessarily stop Celica from being considered.