I actually don't like spending relics (even if they don't have any other use) on something consumable. I would, however, invest them (along with mundane materials) in a meaningful building development, like in Wonders. You could then drop relics from spell manufacture, instead require time or mundane materials... which I would actually consider spending if the reward was worth it. I like lots of the suggestions here but above all I would like to see spells as something you would always want to be producing, which means they have to be built from mundane goods. Alternatively, make lower grade spells (like the current ones) buildable with mundane goods, and "epic" spells cost permanent relics. But if I'm going to build a spell with relics it has to be something I can't get otherwise.
You could, if you felt particularly fancy, have different "schools" of spells within the school to be built up separately through infusion of relics, with increasing potence for those types, eg of categories might be elements, manufacture/diplomacy/war, etc.
As for individual spell ideas:
1) Trade portal - allow trading with distant players for a brief time
2) Fellowship Aid - give neighbourhood help of a random type to all fellowship members that you could manually help. (This would be worth tossing a relic away just to save time).
3) Inspire - knowledge points produced X% faster for a half day.
4) Delve - cast on an already explored province to try for a rune/relic/other random benefit.
4) Fireball - deal X damage to all creatures within an encounter, before begin fight
5) Thunderstorm - cast on an encounter before a fight, deals X damage to a random foe each combat round
6) Regeneration - cast on an encounter before a fight; all creatures regenerate X life each combat action
7) Resurrection - use instead of paying gems for resurrrecting lost units in encounters, because who does that? You can still make spells buyable with gems. A spell should resurrect X people-equivalents of a single/multiple stacks.
8) Opportunity - create a short term new quest - this would have to be worth more in rewards than the cost of the spell!
9) Alchemical transformation - transmute between goods at superior efficiency to the existing wholesaler.
I would also like to say Raistlin's idea would be another, different avenue that would probably be simpler (but I like the idea of building and choosing what to build up more
).
Thanks.
Ash.