I don't know if Kamui being a DLC fighter would be a thing of the present but I do think he has a shot in the next Smash Bros. game. Either way, I'm all in for him/her.
(This is more so this is addressed in this thread, and less at you specifically)
The thing with Fire Emblem is that they rarely have a protagonist appear twice; it's only happened with Ike and Marth, and with Marth it was because he was insanely popular, and with Ike it was likely planned beforehand for 'we can't fit all this plot in one game' reasons.
It's also highly likely that whichever FE game is the most recent is going to be the one represented, and there's no guarantee that there isn't going to be another game between IF and 5mash.
Basically, we can't really be sure that it is safe to wait for 5mash, unless Kamui is intensely popular, which we know firsthand that isn't a guarantee to get in. Granted, Kamui has way more going for them than Chrom did, but the point still remains.
Also why would
anyone we want to wait? Kamui is cooler than Ninian!
that's a pun. Ninian's an Ice Dragon.
But thanks for the support!
Hope I didn't come off as confrontational!