Despite the lack of a Zelda newcomer since Brawl being a obvious point that needs to be addressed, as Zelda is easily one of Nintendo's most succesful gaming franchises and Breath of the Wild was phenominal, I'll try and keep it character-related. Zelda really doesn't have many consistant characters that return to each game, outside of Link and Zelda. Ganon is also up there, but GanonDORF definitely isn't for example. And let me just compare Impa with Ganondorf here.
Impa might not always have a role in the Zelda franchise as a fitting Smash character, being a elder lady in her most recent incarnations (ALBW
and BOTW
) as well as her original design on the NES Zelda game, but her most popular and prominent roles are definitely as a Sheikah warrior who safeguards the princess, Zelda. Ocarina of Time was the first time Impa was actually included in game for example. And here she serves as both a mentor to Link and Zelda, as is one of the Seven Sages. She returned in Skyward Sword with an impressive revival role, as well as a double role which is only revealed at the end of the game, much like
's role was in Ocarina of Time. You see the character often, but only near the end you fully understand what their role was all along. Then, shortly after, she first became playable in Hyrule Warriors as the first character alongside Link. Therefore, she's the first character aside from Link ever fully playable in a "Zelda" game. And she also is a sort of co-lead alongside Link, and features a design merged from both Ocarina of Time and Skyward Sword, as well as being guardian of Zelda yet again.
It might not be all that much, but she's the most prominent Zelda character aside from incarnations of Link, Zelda and Ganon, whom all have their most popular and prominent versions of their character featured in Smash. At the very least she could've been a Sheik Echo, but I believe she can have way more.
Since Breath of the Wild is HEAVILY Sheikah-themed, and BotW being a "template" for future Zelda games, Impa could easily borrow the moves of Link's Sheikah Slate to create a character around. Master Khoga, the Yiga Leader, also uses a few Sheikah Slate magical attacks as Magnesis. If the Yiga Leader could do so, so could the Sheikah Leader. Since Breath of the Wild is bascially the end of the Zelda timeline, and Skyward Sword is the beginning, it's likely a next Zelda game will be in the past of BotW, and since BotW features a Old Impa, a prequel would feature a Young Impa. She has fought against the Calamity in her younger years, and was implied to be a strong warrior close to both Link and Zelda as well as the Champions. She would be the IDEAL candidate for a Zelda newcomer that's added down the line of Smash development, when news of a next Zelda game will probably reach us before they reveal Impa as one of the DLC characters. That's good foreshadowing.