I'd like to see Unova get a visit in Pokémon Z. Why? Unova was, in a way, a "new start". Also, I think there are still a good number of Unova Pokémon that are not in Kalos (if I am wrong, correct me; maybe I'm thinking of the legendaries here). All of the Unova Pokémon can then be found there and other Pokémon not in X/Y added in Kalos itself.
So, how would it connect? Through Lumiose City, the same way you reach the other post-game activities. You must complete an extra mission to unlock the train to Unova.
Speaking of Unova, anyone remember Anville Town? It can somehow be connected through continuing development to another part of Unova, perhaps through a new Route 24 path that finishes with a stairway into Anville Town. The train can get off there, and the other train heading into Castelia can be made unavailable. My thinking is a connection through a path north of the Celestial Tower on Route 7; this can be Unova Route 24. Route numbering conflicts? Just make Kalos Routes start with a K and Unova Routes start with a U; i.e. Route K2 and Route U7.
Oh, and I'd like to see the game start in Lumiose City this time around, sending you through the lab for your first Pokémon. It would be neat to have multiple paths available from Lumiose to start the game, though I doubt this would happen since they want the Gyms to have a progressive order. But seriously, I'd love all alternate paths to require completion of the Lumiose Gym.
The only problem with my idea is where you'd train at the start of the game, and that unfortunately rules out Lumiose as a starting location. It wouldn't make sense to devote one of the loops to the training area, so they'll stick with Vaniville Town or create another new city somewhere.
EDIT: Wait, change that.
Route 4 is where Ash starts outside of Lumiose City in the anime. So Route 4 makes sense. The area beyond Santalune can be opened up until the forest, which can be a post-game event related to my Unova idea and MUCH stronger along with Route 2. Route 1 could have some grass coming up in the middle of a breaking path with new Pokémon there as well, or instead lots of challengers hanging around the area since it's not an opening town now. That could even be the challenge; get to Vaniville Town for your pass to Unova from an Unova character who has moved there. Route 22 is still there as well leading to Victory Road, but Route 21 can remain inaccessible. All of Lumiose is open and perhaps a new wild grass area can slip in somewhere like in Castelia City.
Personally, I'd love to see an auto-save feature in their next game. Every time you reach a new checkpoint (such as defeating a gym leader, entering a new location, obtain some important event item), the system would save your game in the background. :] Thanks to that, you wouldn't have to worry much if you ever forget not to save. Another thing I'd like to see them do is bring back Vs. Seeker.
No, no, no. No auto-save. Sometimes people want to bum around doing whatever they want for fun and don't want the game saving their messing around.