One thing I've realized lately, is that it seems like there'll be at least a little overlap between the GT Cafe collections and the various campaign events, with the French Hot Hatches motif being one such example. I could see there being similar motifs where there's both a collection and an event, especially if these events from previous games return:
Race of the Red "R" Emblem
Nissan Skyline GT-R (KPGC10)
Nissan Skyline GT-R (R32)
Nissan Skyline GT-R (R33)
Nissan Skyline GT-R (R34)
Type R Meeting
Honda NSX Type R '92
Honda Integra Type R '98
Honda Civic Type R '98
Evolution Meeting
Mitsubishi Lancer Evolution IV
Mitsubishi Lancer Evolution V
Mitsubishi Lancer Evolution VI
Mitsubishi Lancer Evolution X Final Edition
Supra Legend
Toyota Supra (A70)
Toyota Supra (A80)
Toyota GR Supra (A90)
Z Heritage
Nissan Fairlady Z (Z33)
Nissan Fairlady Z (Z34)
Nissan Fairlady Z 300ZX
Silvia Sisters
Nissan Silvia K's
Nissan Silvia Q's
Nissan Silvia Spec-R Aero
Stars of Pleiades
Subaru Impreza STi 22B '98
Subaru Impreza WRX STi Type R '99
Subaru WRX STI Type S '14
However, with all this in mind, I'm not sure if there'll be collections that will feature cars all from the same automaker, or if most (if not all) of them will try to feature as wide of a variety of manufacturers as possible. I do know the Kei Car collection has two Honda models as well as the Daihatsu Copen, but even then, that's not the same as something like a "Type R" collection, which would entirely feature Honda models.
I could see the collections having solid synergy with the campaign events, where if you buy a car to compete in a given campaign event, you've already begun to work towards completing a given collection. Maybe you could even have a random prize car that fits into the event's motif, and would also work towards the collection.