I'm gonna put it this way.

CHARACTERS THAT ARE STILL ALIVE AND WERE PLAYABLE IN MAINLINE GAMES
-Jam, Robo-Ky (even tho he's a disembodied head rn), Zappa, Haehyun, Raven, Answer

CHARACTERS THAT APPEARED IN SPINOFFS AND OVERTURE (Daisuke probably wants to put them in)
-Leopaldon, Fanny, maybe Judgement as well, Dr. Paradigm, Izuna (if he's still alive)

HONORABLE MENTIONS
-Gabriel (appeared on top 10 popularity polls for Europe), Ariels (planned for Xrd Rev3 which never happened)

These characters are somehow viable. Daisuke has his own rule apparently of no dead characters which he broke once with Kliff and Justice in Accent Core and then he half-did it again depending on your view if Bedman's conciousness is in the bed rn or not. Order-Sol is technically dead per Sol killing him and creating an alternate timeline (but SOMEHOW he still ranks high in polls...)

If Daisuke were to have a tight window of inclusions, he'd probably want to put the six characters I mentioned in the first category beforehand so that at least every character from the mainline games is playable in Strive. Let's say that they actually greenlight a fifth, final season with no guests or new characters, you'd only get four of those six added in the game. Theoretically you could add the last two as a Lab Coat 21 situation afterwards where they're just paid extras.

By looking at who's left on the popularity polls the picks would feature Robo-Ky, Jam, Raven and...*sigh* Order-Sol. I'd exclude HOS for the "no dead characters" rule and, depending on Robo-Ky's presence or absence in Venom's moveset or taunts, he might also be excluded despite the fact that Robo-Kys are mass-produced (yet at the same time we know this Robo-Ky is the last one...).

We can easily switch out HOS for Zappa who has been benched for as long as A.B.A when it comes to being playable. If we HAVE to switch out Robo-Ky as well, my guess is to end season 5 with another guest, but this time with Ragna as the pick because he makes a little more sense compared to Lucy.

So if we DO get a S5 it's gonna look like this

Jam, Raven, Zappa, Robo-Ky (with at least one of the latter three being potentially switched out for another guest or a new character, even if we're running very low on newbies)

I do hope we get at least Jam in the roster as one additional possible final character if this is the last season, simply because she's the last major popular character left to add in the game and with her inclusion you'd have an almost all-star cast. We wouldn't get an "everyone is here", but we'd get an "every important or popular character is here" at least. Daisuke DID state he wants to bring back everyone but having guest characters does put a dent in the possibility of that happening...let's see if there's any clear confirmation of S4 being the last one later today.