time travel has been a part of the KOF canon for a while now, so it's not an issue
Wait, time travel is canon? I thought all those dead characters returning and all that was just dream matches! Guess I gotta read up what happens after the Orochi saga, lol.
But, okay, I'll concede. Guest characters in the context of the thread specifically refers to third party characters, which I don't think the Street Fighter series needs since guests are usually added just to help give more visibility to the franchise hosting them, and Capcom, if they ever need to boost attention towards their games they can just add costumes from their own IPs, they also look super awkward sometimes, like everything they have crossed over with in Monster Hunter.