Curtail the power of lower level spells by implementing caster level caps
TRUE SEEING. It shuts down an Entire School of magic if you have it.
There seems to be some synergy here.
I personally don't have a problem with the core idea of True Seeing being ROCK to Illusion's SCISSORS. Particularly considering it is a 5th level
cleric spell. Taking out a 6th level spell with a 5th level spell that is intended to do just that, is cool. And a 7th level spell, frankly and imho, is also completely fair. 8th level? Well, pretty far off in subjective land of course, but for me, I'd think an 8th level spell might reasonably get a fair shake at shrugging off the 5th level counter. And 9th level should have a very solid chance of ignoring it.
So perhaps some sort of opposed check is in order. Just as a starting point I'm thinking opposed caster level checks with a modifier of +5 / spell level going to the higher level spell.
I think this could be a very simple back pocket rule for every case in which two spells come into conflict. Just having this kind of mechanic in place
might facilitate creative spellcasting. For example, can you dispell/destroy a wall of ice with a fireball? Ok, but it an opposed CL check, with a -5 penalty. Or block a fireball with a wall of ice? How far you could go with this would depend on creativity and what the DM can accept. I think trying to flame out a wall of fire with a fireball (like putting out a burning oil well) would be clever, but using a fly to divert a fireball harmlessly away wouldn't cut it for me.
Obviously some scenarios require ready actions and may suffer lack of implementation (as counterspell often does), but others offer chances to face off against on-going effects and could be quite dynamic.
The whole thing begs to create a "wizards duel" system even.
Imagine dueling illusions in a battle whose outcome had real significance. You are a mighty warrior and you watch as sweat pours down the face of you wizard buddy while his illusionary knight goes toe-to-toe with the evil overlord's illusionary demon.
In the case of something like True Seeing, a +15 bonus is applied to appropriate checks.