There will be future generations. Here are a few reasons why:
1. There hasn't been 16 Eeveelutions yet. We have yet to see a Poison, Bug, Flying, Dragon, Steel, Rock, and Fighting Eeveelution. THere will need to be future generations for that.
2. As for Arceus, I have two words: screw it. Wasn't Mew the original Pokemon god? Besides, there will always be different storylines and plots.
3. New styles of gameplay are being developed out the wazoo. If they introduce a party system where you can team up with other trainers, then the window for more games will open wider.
4. GSC did well. THey did a remake of RBY, so there's gonna be a remake of GSC, as will RSE.
5. There will be more RPG games for consoles, such as the Wii. With these in mind, they're gonna need a boost in revenue after the (in my opinion) epic failure of PBR. I like battling, but I also like adventure.
These are five good reasons why there will be future generations. Reason #1 is one that makes a lot of sense to me.
Also, a lot of Pokemon look like they can evolve again or have pre evos. For example: Absol, Tropius, Miltank/Tauros, Jynx, Delibird, Skarmory, Chimeco, Banette, Breloom, Manectric, Zangoose, Seviper, and many others.