I'd rather grant the caster of
Detect Magic an automatic Will save for their spell to let them see past the inherently deceptive nature of the illusion. (Applies to Illusion spells that allow such a Save, obviously). Fail the Save and the Illusion fooled you, even through your
Detect.
That way
DM would still detect magic items based on Illusion magic (a valuable tool), but the caster would stand the same chance of being fooled by actual
Illusion spells as anyone else. (Okay, the caster has a better chance of spotting the
Illusion, simply because they get the save automatically, but hey, they paid for that when they cast the
Detect. It ought to be worth
something.)
For
Invisibility spells, I'd have it reveal that there is magic in the area, but treat it as if the magic were not in a line of sight, so they can't pick up the school, and perhaps even stop it from advancing past what you can get on the 2nd round of the
Detect. That is, you know it's there, somewhere, you know about how strong the magic is, but you can't quite zero in on its location or type.
To me, that's a fair balance between nerfing
Detect Magic and nerfing the various Illusion spells.
Note: This doesn't address the various
Illusion spells that
Detect Magic doesn't help with, like
Blur and
Displacement. I think those work fine just as they are.
Mirror Image is an odd one. By the rules the caster of the
Mirror Image can only change places with his decoys on his own Move action, so once you know which are images and which is the real McCoy, you can direct your allies to focus on the real caster, effectively negating his defensive spell until he can play his shell game again on his turn.
And you know, I'm okay with that. The one caster is doing nothing, offensively or defensively, other than concentrating on their
Detect and pointing the finger, while the
Mirrored caster is still free to take offensive action as they see fit. Such as splattering that annoying tattletale who keeps blowing their cover by pointing the finger.
Detect Magic may be useful as hell in non-combat situations, but once battle is joined actions are at a premium, a precious and limited resource. For a spell caster to blow all battle prep, buffing, defensive and offensive casting in order to maintain their concentration seems like a really poor allocation of resources. And I never interrupt an opponent when they're about to make a mistake.
***
Just had a thought, a "Plan B" sort of solution to issue.
Change the duration on
Detect Magic to 2 rounds, plus one round per caster level.
While I would personally be suspicious if a player seemed to be able to pop a
Detect an endless number of times, or always seemed to have one ready to go no matter how many times a day the occasion came up, this change would highlight that sort of abuse, make it harder to slip under the radar.
On the other hand, it might encourage even more of that sort of cheating.
