GT Karting Series (Beginner)
-Permitted cars: GT Kart 125
-Courses: Red Bull Short, Suzuka East, Tsukuba Circuit, and Maggiore - Center.
BMW i3 One-Make (Beginner)
-Permitted cars: BMW i3
-Courses: Nurburgring GP, Tokyo Central (wet), Sardegna C, and Suzuka East
Homologation Specials (Amateur)
-Permitted cars: Alfa Romeo Gr.3 Road Car, Chevrolet Corvette Gr.3 Road Car, Ford Mustang Gr.3 Road Car, Mazda Atenza Gr.3 Road Car, Mitsubishi Lancer Evolution Final Edition Gr.B Road Car, Peugeot RC-Z Gr.3 Road Car, Subaru WRX STI Gr.B Road Car.
-Courses: Dragon Trail - Gardens, Tokyo Expressway South, Willow Springs - Big Willow, Mt. Panorama, and Sardegna B II.
Group C Challenge (Professional)
-Permitted cars: Mercedes-Benz Sauber C9, Jaguar XJR-9, Nissan R92CP, Mazda 787B, Porsche 962.
-Courses: Fuji Speedway Short, Circuit de la Sarthe (no chicane), Monza (no chicane), Blue Moon Bay - Infield A, and Maggiore - GP.
Gr.4 Endurance Series (Endurance)
-Permitted cars: Gr.4
-Courses: Sardegna B, Tokyo Expressway East, Circuit de la Croix A, Nurburgring GP, Mt. Panorama, and Brands Hatch.
EDIT: I think we'll see more events get added to GT League when there are less events that have under seven rounds. While the J-Sports Meeting is an exception, it doesn't seem likely a given event will get over seven rounds. But I think there are a decent amount of events at the moment that still have room - perhaps this could give us clues as to both what rounds could be coming to which events, and maybe even what sorts of cars could be coming.
For example, the Nissan GT-R Cup in the Beginner League still has room for two more rounds, so another GT-R could be coming. If it's anything like how the Z Heritage event permits the Greddy Fugu Z, maybe even a tuned GT-R could be coming, such as by Nismo, Autech, Mine's, or some other tuner. Even within Nissan, we could see one of the older R35 iterations seen in GT6, such as the original R35 from 2007, the Spec V, or perhaps the Black Edition which also had the R35's first facelift. There's also the KPGC10, as well as the C110. Unlike the "Race of the Red Emblem" event from prior games, I don't think Skyline models without the GT-R badge (e.g. the R30 and R31, the older BLRA-3, or the models known in the US as the Infiniti G35, G37, etc.) will be permitted into the GT-R Cup, so it's a bit harder to guess if those sorts of models could be coming - though I don't think they're necessarily any more or less likely to appear in DLC, just because they wouldn't qualify for this particular event.
EDIT2: It looks like the Z Heritage event only has three rounds, so I could see that event being even more likely to get new rounds. Especially if we see the Z34 from the PS3 era. Like with the GT-R, we could see tuned Fairlady Z models, especially given the eligibility of the aforementioned Fugu Z, but also the original S30, among other previous models. But also like the GT-R Cup, I don't see this event making it notably less likely for a non-Z Fairlady model (like the Fairlady 2000) to appear.