The Season 4 of Guilty Gear Strive marked a bit of a historical moment for the series. For the first time since Guilty Gear's inception the game featured a guest character. In this case, Lucy from Cyberpunk: Edgerunners (which is getting another season, in case you haven't heard already).
It seems like Arc System Works doesn't plan to stop with the crossovers, so they have just announced a second guest character that will be arriving in the Winter of 2026.
Naturally, the speculations are all over the place. Perhaps bolstered by the recent BlazBlue news, there are many posts guessing that Ragna Bloodedge will be making his way to Strive. Another guess assumes that it could be someone from RWBY, which is coming back for another season and has previously collabed with BlazBlue: Cross Tag Battle. Even the font choice was enough to make people think that we're getting someone from Granblue, Melty Blood, or Final Fantasy XIV.
That alone should tell you how vague this all is, and consdering that the previous guest came from Cyberpunk, pretty much nothing is off the table.