Since there are six spots in the Career ladder that say "COMING SOON" and one of them is already confirmed, I'll say what I'd like to see in the remaining five (noting that Indy Car is most likely Tier 2):
Tier 6: Aussie Racing Cars
Tier 5: TCR International Series
Tier 4: V8 SuperTourers and Indy Lights (you could probably "ghetto" Indy Lights by just running the Formula C around ovals, though)
Tier 3: Super GT (I can dream!), more realistically: V8 Supercars
I think this would nicely flesh out the Touring ladder a bit more nicely, rather than it getting slotted as a lower category before the GT's. Also some Oval Racing in there because why not.
It's still way too early to speculate, but I think these series are going to be the most likely to appear, based on what is in development at the WMD forums:
Tier 5: Some Touring Car series of some kind (I think Sector 3 has an exclusive deal for WTCC though, so that's probably right out)
Tier 4: Renault Megane Trophy
Tier 3:
- NASCAR Sprint Cup (doesn't this also have an exclusive deal though?)
- V8 Supercars
- Le Mans Series
Some of these series would need more appropriate tracks to race on, but I'm making the assumption that will be a thing SMS will do if either my speculation or wishes are true.