A known leaker has spoken about when we might see a Resident Evil 9 release, and it's a lot sooner than you might think.
According to Dusk Golem, who was asked 'Should we expect Resident Evil 9 next year hmm?":
"2025, I'll say that much, though probably will be announced next year. I won't answer any further RE stuff, but I'll throw that bone out there. There's a big Capcom game being announced at the end of this year that's releasing in Fall 2024, and likely RE9 will be announced next year and release 2025"
This would line up with recent Resident Evil release schedules which appear to have Resident Evil Remakes between the new mainline titles. For example, between Resident Evil 7 and Resident Evil 8 we saw the release of Resident Evil 2 Remake and Resident Evil 3 Remake, while between the release of Resident Evil 8 and Resident Evil 9, we've already seen the release of Resident Evil 4 Remake.
As to what the unannounced Capcom game that's reportedly being announced at the end of this year could be, it's really anybody's guess. Street Fighter has already received a new game and beyond another Resident Evil Remake, we're not sure what it could be. Perhaps a new Marvel vs Capcom could be coming?