I do not expect combined showrooms for the mainstream brands. To me it makes no sense to put a Opel Corsa, Peugeot 208, Citroen C3 and a Fiat Punto in one space.
PSA/Tavares could have brought the Peugeot and Citroen brand dealernetworks together for more than 30 years and they've always kept them seperate. In my opinion for good reason. All these brands will again become/remain full line brands. If you cannot afford to have a full line of vehicles for all your brands it makes sense to combine them in one showroom, this happened to some success in the US with the CDJR dealerships.
Now in a stronger position you see FCA trying to move to seperate showrooms for Jeep. They started since 2017 with pushing for seperate Jeep showrooms in all non-rural markets. When the Wagoneer was presented they said that only dealers with seperate Jeep showrooms would be able to sell Wagoneers it was 1 of the conditions. They said at that time there allready were 600 seperate Jeep showrooms in the U.S..
I think this expansion of seperate Jeep showrooms will give room to finally get additional nameplates for Dodge, Ram and Chrysler. Jeep right now is at 10 nameplates and more are coming. CDR right now are at 9 nameplates. (Toyota 16, Ford 15, Kia 12, Chevrolet 19, Hyundai 13, Nissan 18, Honda 11, VW 9, Subaru 8)
Peugeot, Citroen, Opel and Fiat all have more than 10 different nameplates in their showroom they compete directly with each other.
You must remember that people and businesses buy an Opel Combo because it's a German car with German engineering and workmanship.
Combining them in one showroom would be the fastest way to lose a great deal of their current marketshare.
For the premium brands you are talking right now about 7 nameplates in total (4 DS, 2 Alfa, 1 Lancia). If all goes well this number will max out in 8 years in about 16 nameplates. The number of showrooms will be relatively limited and i think they will be able to give each of the 3 brands sufficient room to grow together keeping their own identity.