...how would you feel about them instead being from either the Sol Dimension or from Silver's future?

Both areas are ripe with potential to explore, and allowing a more dense roster of characters within each of these worlds further opens the doors for spinoff potential more if they become fan-favorites with the casual crowd.

Both characters have some history with either Blaze or Silver in the comics anyway, so their dynamics would still be ones fans can look forward to, and considering Sonic's world is already full of characters that need more love anyway, I don't think it'd hurt to throw the fresh blood in the direction of the two characters who always need a contrived method of time or space travel just to involve themselves with another character. Let's see these guys get their own entourages, and what better way to do it than with characters SEGA already knows people would accept with open arms?

It wouldn't be perfect, as that would require faith that SEGA consistently remembers these places/characters exist and can be explored again, but I think it'd make for an interesting alternative to the comics without deviating too much from what made Tangle or Whisper great to begin with.

This could apply to any character from external media, really, but Tangle/Whisper are fresh on my mind due to their unprecedented incorporation into Speed Battle.