X999 is pretty much just the catch-all for absurdly fast cars, so that's not the limiting factor. It's more having to navigate around who holds the rights for those series and whether there's any exclusivity to them. EA/Codemasters has F1, iRacing has NASCAR, and potentially we could get IndyCar after the whole debacle with Motorsport Games sees some kind of resolution, but I'm not really banking on it anytime soon.
T10 could theoretically backdoor it by licensing cars direct from the teams, as has been done in the past, but that's a whole cage of rats in itself and I get the feeling they'd rather focus on touring series like WTCC and Trans-Am with this iteration.