Tetra was never among the Forbidden 7. It was blatantly and intentionally a toon clone of Sheik. In the end, yes, Sakurai probably would've eliminated the clone idea and went for Tetra, but that depends if he had the time. We have the facts, and there is nothing whatsoever to suggest Tetra was ever even considered once for Brawl. The file name specifically says Toon Sheik for a reason, and it's pretty blatant what it was. It was pretty clearly just a straight clone of the duo, just like Toon Link is for Link(with mostly aesthetic chances and a few effect changes, rarely any move differences otherwise). Tetra had no actual real moves until Hyrule Warriors anyway, so her potential was less than a clone of a character with an actual moveset. It made sense what he went with as an idea, being clones were easy padding to a roster.
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Anyway, Tingle, Impa, and Moblin Ganon are currently the most viable options we have. Sakurai has shown he really doesn't take one-shot characters into account unless they're the Triforce Trio, and he clearly considers recurring characters to be a lot more important. I wouldn't even put any of these 3 with a very good chance either. Impa is entirely inconsistent design-wise and he couldn't even be bothered to mention her fully till Smash 4 with just some trophies. He blatantly has ignored Moblin Ganon since forever(he hasn't had a trophy or sticker, with at best a song referencing one of his battles). Tingle's bigger issue is creating a fun moveset(compared to the other 2), but notably still has a large hatebase(not as large as it once was, due to Hyrule Warriors as well as not shoehorning him into Zelda games with poor roles like WW did), enough to make him unlikely.
Zelda newcomers don't have a large chance at this point. In addition, besides Tingle(who he spoke on), all we've seen so far is the Toon Zelda/Toon Sheik information, and being he already incorporated Toon Zelda's only unique gimmick into regular Zelda, they're likely eliminated as ideas. For the most part, Tingle is about the only candidate that hasn't been entirely eliminated but at least thought on. We can hope others were considered, but there's nothing to really suggest that is the case.