Yeah, the key is expansion, not piling on more of the same. That's why your temperature system is such a great idea; it expands, adding more potential to Pokemon and how their moves affect others.
I just realized something; would the temperature system affect the Pokemon only or would certain moves have a colder/warmer temperature, thus having a different effect in certain cases?
Regardless, they really need to expand on the ideas already in existence. Pokemon Battle Revolution was adding random battles, motion-sensor control, and a lot of annoying Colosseum rules. If they had expanded (or even included!) minigames, it would've been worth the $50 I spent. Think about it; they could've added motion sensor to the minigames, possibly a new rendition of "Clefairy Says"; instead of pressing the Control Pad directions, you move the Wiimote up, down, left, or right. That's just one case in which they could've expanded. I'm not a huge fan of only expanding a game with motion sensor, so I wouldn't approve of that being the only change, but that's certainly a start.
They should take the same sort of concept and apply it to Pokemon. Again, not just a touch screen interface, with things like Poffin mixing using the Stylus, or Chatot utilizing the Microphone...expanding upon the actual game. I suppose they figure the little kids they direct Pokemon towards these days are better suited to move a stylus than they are to be able to understand a new system. That's probably a good portion of the reason that they haven't expanded the game a whole lot since game one. Maybe Nintendo would find that if they geared their Pokemon games towards some older people (as in teens-20s, not talking about more of the "all ages" concept) they would have more customers in that department.