Well many FE characters in smash may not have any future in FE (outside of DLC ) either.
I don't think the future thing should apply to Zelda , more like which characters have lasting popularity/appeal like Midna. Arguably, after Impa there isn't really anyone who is on par with the 4 main characters so eventually much less important to the series as a whole characters would be added, perhaps.
To be fair, "lasting popularity" is a pretty vague idea, that is diffcult to determine immediately after the conception of the character.
Sure, in hindsight we may be able to qualify who fits into these character, but at the same time, Zant was popularly wanted for Brawl, and look at how common he is now!
Zelda is a great demonstration of what the Pokemon franchise is not; characters who are merely most important and who make the series so fantastic. The idea goes back to the original premise of SSB, the Nintendo All Stars- after all, there needs to be SOME criteria for a characters inclusion, and as we can see, anyone *could* be unique. Under such light, every popular character cannot get in, and there are many popular characters- so merely being "popular" and "cool" to a group of people just, numerically, cannot be enough. (Except of course in the case of Pokemon, but I think this is due in huge part to TPC).
Not to mention, the nostalgia factor is very high with what makes a Nintendo character 'popular.' Whichever characters an individual grew up with are likely given a boost in the 'desire' department. The issue within Zelda is that often these are characters who are never seen again (and, only influenced a single storyline).