Topaz and Stone need to be introduced to the main canon, if not permanently, then even just briefly for a few games.

I am usually an advocate for keeping different forms of Sonic's media as separate entities/universes (sparing IDW since it provides a lot of context after Forces ended), but Topaz and ESPECIALLY Stone add so much to the world SEGA has created for Sonic. I think that especially with the inclusion of Sage, Stone could have a lot of utility in helping develop Eggman's newfound family unit. It would help add a lit more dimensionality to not just Eggman's character, but the social dynamic between the heroes and villians. Stone and Sage have given Eggman new life that we haven't seen in years and think that can only be a good thing. Eggman has potential to be so incredibly complex and adding a social dynamic amongst himself and his allies could really escalate that.

I have a much weaker argument for Topaz as she doesn't really add much to anything other than Rouge having amazing banter with her, but she's one of the few X original characters that are universally pleasant in all regards, and I really like her. I obviously wouldn't want her as part of the core cast, but if we get another game with human NPCs and side quests like Unleashed and 06, I'd love to see her make a cameo.

Also, since both animals and humans coexist, we could really use some cool interspecies friendships. Elise is really the last new human character to have a "bond" (if you even want to call it that) to any of the core characters, and then of course there's Maria, but she's dead. I guess you could count Professor Pickle's relationship with Amy, too, but even then, it's scarce.

While their inclusion is unlikely, I think the games would benefit from them from a world building perspective. Plus, I just think they are cool characters.