If Zelda is to get another representative, the options as has been said before are Impa, GHIRAHIM, Skull Kid, Tingle, and Vaati. Not much else seems even possible.
Impa Pros:
-Has appeared multiple times
-Most recent appearance is rather recent
-Is a female character (really not much of a pro probably in Sakurai's eyes, but there isn't much to go on)
Impa Cons:
-Only appears as a supporting role everytime, almost impossible to change
-little to moveset potential relying on original moves almost entirely
-Small fanbase
-Potential to be Sheik clone or a luigified clone of Sheik
Skull Kid Pros:
-Really friggin cool
-Main villian role
-Absolutely zero chance of being a clone
Skull Kid Cons:
-Smallish fanbase
-Hasn't appeared in a long while and unlikely to reappear
-Moveset although easy would have to be fairly original due to all of the moves would be derived from instances with his boss fight in which he is always something other than the Skull Kid
Vaati Pros:
-Several main villian appearances
-
Excellent moveset potential
-Little to no chance of bearing any resemblance to anyone else with moves and playstyle
Vaati Cons:
-Small
ish fanbase
-Potentially unlikely to reappear again (???)
Ghirahim Pros:
-Freshest appearance next to impa
-Lead Role
-Excellent moveset potential
-VERY Fitting character
-Fabulous
Ghirahim Cons:
-Unlikely (???) to reappear again
-Smallish fanbase
If one has to be added, as you can see, that if you were to measure the pros and cons on a scale, Ghirahim would clearly have the best ratio.
It could also be noteworthy to say that all of them have the con of having a not so loud or adequate fanbase, so one could say that if we are going on the assumption there is to a newcomer at all, that
that particular con is irrelevant on all of them, and therefore, Ghirahim only has his (potential) oneshot appearance against him. As a result, the only real hurdle for Ghirahim really is whether the Zelda franchise is getting ANYONE. Which is arguably quite the big one, and thus his chances are still low, and everyone else's lower still. (Vaati is a close second though, I find him quite interesting myself.)
I didn't include Tingle because I have said so much why I wouldn't like him appearing in the Zelda franchise a lot already, and see him bearing more potential as his own thingy.
Edit: As for Ghirahim's remaining con, I can see the unlikelihood of Nintendo ever returning to him, or giving him another reincanartion, but I am slightly less harsh on it (and perhaps in a biased manner, but I am saying this point in defense of his con) is that while Nintendo just seems like the kind of company to not take advantage of this, Ghirahim would work
very well if future Zelda games were willing to include the anti-Master Sword again, which is very well possible if Demise is to reappear. In fact, I'd estimate that there is almost a 100% of Ghirahim returning if Demise does. Skyward Sword does indicate that Demise was obviously not always sealed and that there were Link reincarnations before the one in the timeline. I think an exact number was given, 2, which is an interesting thing to note.