So this is interesting: despite Spa being added, the X2019 Endurance Series, Nostalgic 1979, Real Circuit Tours, and Group 4 Cup all get a round at the dry version of Spa, despite the wet version also being available. It makes me wonder if maybe some rounds with the wet version will come in the next update? Notably, Red Bull wet is the other new round added to the Group 4 Cup, and Monza is the other one for Real Circuit Tours.
Funny enough, the background image for Real Circuit Tours features the outline for circuits that aren't actually featured at this point in the event: Red Bull Ring Short, Le Mans, Mt. Panorama, Streets of Willow, Horse Thief Mile, Nurburgring GP, Tsukuba, Suzuka, Brands Hatch, and Nurburgring Nordschleife. It's especially interesting since it appears this event last appeared in GT4, though there also seems to be a very similarly-themed event in GT6, albeit under a different name.
At any rate, I could see the dry version of Spa being reserved for the Gr.1 and Gr.3 endurance events, whereas the wet version would be seen in the non-endurance versions, unless PD plans to break the observed trend of not including wet courses in the endurance league events. Alternatively, the wet version could still appear in the AMG Grand Prix and F1500T-A events, and maybe even the respective events for the X2014 Jr. and X2014 Standard, though I have doubts since I don't recall there being a World Tour event at Spa that used either of those two; I only recall Spa hosting a race with the X2019, as well as a trend of all three of these events being largely modeled on World Tour events, such as with the X2014 Standard round at Le Mans.
Nevertheless, I anticipate more rounds for the World Hatchback Championship and Lamborghini Cup, given the newly-added cars, and the potential return of GT6's Electric Circuits.
EDIT: Perhaps more specifically, the wet version of the Red Bull Ring Short Track could be another World Hatchback Championship round, while the wet version of Spa could also become another round of the Lamborghini Cup. At any rate, it seems like the J-Sports Meeting (and I think the Sunday & Clubman Cup) is the only exception to most GT League events having a maximum of seven rounds. So while the idea of an event obtaining more rounds when it already has seven is technically possible, I would still say it's remarkably unlikely that more events will join the J-Sport Meeting's status. This could be why some events may not get rounds at Spa despite seeming like likely shoe-ins, (e.g. Gr.1 or Gr.3 events) as they've already obtained this seven round "maximum."
However, while this would make the Gr.1 Cup in the Professional League unlikely to get additional rounds (let alone at Spa), the endurance events for Gr.1 and Gr.3 are only added as one at a time, and they have six each, so there could still potentially be rounds at Spa for these events. The Gr.3 Cup in the Amateur League is also below the limit, with five rounds so far. But I also don't see that many events focusing on a particular model (e.g. Z Heritage) getting more rounds even if they have less than seven, unless more cars like that are actually added in an update.
For example, I don't foresee the Nissan GT-R Cup, Z Heritage, or Supra Legend events getting new rounds unless, for example, the KPGC10, Z34, and A60 respectively were added in future updates. I'm also unsure how more tangentially-related models could factor in, like if a TRD 3000GT or Celica XX would be eligible for Supra Legend, or a Nismo-tuned GT-R or Fairlady would be eligible for those events. It does seem like the Greddy Fugu Z is eligible for the Z Heritage event, though, so that could be indicative of things. This would also make sense for if the events return in a game that has more tuning options, like aero kits and carbon hoods; why indeed should tuner variants be excluded, in that case? (I do think it'd be neat to have more events that prohibit tuning otherwise, though - I think some GT6 events did that. Or maybe an event that only allows you to change the downforce, or maybe events involving a "major" group could typically have BoP on for all racers, including the AI, like for a Gr.3 event. There's a lot of fun you could have with regulations, and I'd hope that more interesting regulation options can also be an option for lobbies, as well as something seen in Sport Mode races. I could see regulations having enough depth to be worthy of being discussed and shared almost in the same way tuning settings are.)