Here are two quotes that suggest there might be a possibility of what you are proposing regarding the XP gain from casting spells. In other interviews, Todd has mentioned that you level faster using skills that are higher level, so I imagine if your Destruction is at 90, for example, when you cast a fireball, you get more XP than casting a Paralyze spell if your Illusion skill is only 10.
1. May 2011 Norwegian Gamer.no Interview Q: In previous games many skills were increased by casting the "unlock" spell fifty times on a door, and so forth. Is this still possible?
Todd Howard: No, we have resolved most of these issues. Some of the skills we have, for example, gotten rid of, like "Athletics" and "Acrobatics". For who creates a character and think "I'll role-play a guy who runs"? Much of the repetitive activities have been removed.
2. April 2011 Ausgamer interview:"Your skills are still levelable through active use, in that performing said skill will advance it. However, you can’t just spam them now, as the active levelable skills require specific use. [/size]So, essentially, you can’t just stand on the spot and spam jump to level your Acrobatic skill, because it no longer exists.
It’s a great system that requires consistent and active play and engagement with the game , which is something the team strove for from Day One.
Moreover, skills are now actively tied to levelling your character, and not the other way around. It’s slightly complex, and Todd explains it better than I could, but we have a Q&A transcript coming today, so stay tuned, but it makes much more sense than the system in Oblivion, with the perk system basically pulled from Fallout 3 to create what seems to be the most logical character-levelling and progression system ever created. "