Since we are brainstorming anyway, I think having the season/environment affect spellcasting is a great idea. This could easily be taken a few steps further:
1. Cold based spells get a bonus when used in a cold weather environment. (The inhabitants of Everfrost would likely have higher cold resists whereas critters in the Thundering Steppes would likely be weakened.
2. Fire based spells get a bonus while in a hot environment. (See the possible environmental consideration in suggestion #1.)
3. Disease spells would get a bonus in a jungle/marsh/swamp environment.
4. Nature type spells get bonuses durning spring.
5. Death type spells get a bonus during fall/harvest.
6. The subterranean races (Dark Elves, Ratonga, Dwarves, Gnomes, etc.) get bonuses at night, penalties during daylight.
7. Surface races get bonuses durning the day, and penalties at night.
8. Humans get no bonuses/penalties for time of day.
(It might work better for balance reasons on PvP servers to give darkies bonuses at night and lighties bonuses during the day regardless of race. Would make a darned interesting dynamic.)
9. Rangers get stealth/track bonuses in the wilderness whereas assassins get stealth/track bonuses in towns/cities. (Then put some dark alleys in the cities where assassins can lurk around to prey upon the unwary.)
10. Make necromantic/evil inquisitor spells more powerful at night and cleric spells more powerful during daylight hours.
Some of this may be in play already as far as I know I was just throwing out a buncha suggestions which would have the overall effect of forcing players to be more aware of their environment.
-Sam