I would love the Ferrari 550 GTS to return but the reason it didn't make it to FM4 was licensing. Its not really a official Ferrari product. It has some Ferrari blessing to exist although I'm guessing Frederic Dor had to pay a pretty penny for that to happen. I'm guessing the same licencing issue still remains. Unless theres been a change of person responsible for these decisions and a change of heart.
Maybe the 575 GTC which was official Ferrari product could appear?
Either way would love to hear that V12 scream again around Sunset.
Considering they'd likely have to rebuild the 550 from the ground up at this point, I think they could just go with the 575 GTC if they wanted to give us a GT1 Ferrari. Problem is, why would they? Early 2000s GTs are completely absent from FM23, and there doesn't seem to be any interest in reintroducing the class. Even the Aston DBR9, which kept us company through the Xbox One era, is gone. The 575 would be a fish out of water, stuck competing with the GT500 Nissan R33 and a variety of older road versions of late-90s GT1s.
@RolexGTP also mentioned the Carma 308 - I don't see that ever coming to FM for the exact same reason as the Care Racing/Prodrive 550, it was not developed by Ferrari.
I'd say the most likely candidates for a Ferrari month would be the 1966-7 Ferrari 312 to bring the 60s Formula class closer to completion, and the 288 GTO Evoluzione (which, IIRC, showed up in the Dead Sea Scrolls back in the FH4 days -
@TheAdmiester can probably confirm or deny that) to join the field of Group 5 racers already present in-game. The 312P last seen in FM7 would likely make a return as well, as it would be a good opponent for the Matra we were recently blessed with, the Porsche 917 and any 908 Spyder variants they may add during Porsche Month.
But of course, the star of that update would be the inevitable 296 GT3, perhaps the 499P if it can actually be licensed (and in the worst case scenario they can probably get cheeky and add the Modificata instead, with an engine plate downgrade that magically brings it in line with the WEC version's power output, or in any case closer to the other Proto-H cars).