Having a discussion with one of my players who was looking at developing a new version of the mirror image spell. He wants to have a equivalent spell that he can cast on another person (rather than on self). I cried (on the inside) as I thought about such a spell being cast by his 16th level wizard on the party 'tank'. But I thought that before I started making things difficult for him, I would ask the collective community of wisdom, if someone has already come up something like this, either in a Pathfinder product that I don't have (yet) or in a third party product?
I've only rarely put on the DM's hat, but I would probably let him. Adjusting the parameters of the spell might mitigate some of the problems you are foreseeing.
Making it a level 4 spell or above would prevent making it into a potion or quickening it with a minor rod, so it's a steeper investment in terms of either action economy, spell slots or gold. An increase of two spell levels compared to the original spell might warrant extra goodies, though - maybe give the target the option to "spend" mirror images to get a flanking bonus, sending one of the images ahead to menace an enemy or what not.
Another option is to keep the spell at level 2, and as a trade-off for making it a targeted spell, reduce the number of extra images per caster level or something. Making it a touch-range spell could complicate its use, if the wizard has to move up to the tank to cast it on him. I think this is the less interesting option, though.
There are a range of avenues for CR 16 opponents to get around the mirror image miss chance (monsters with truckloads of natural attacks, things with true seeing or that sort of thing, blind-fighting, dispelling, area of effect, etc.), but if the wizard does put the effort and resources into researching this spell, it should not be wasted either. Where to tread that line is something more experienced DMs might have ideas on.
At level 2 you have Blur, at level 3 Displacement, which can be cast on others.
The Mirror Image spell (like any other Personal spell) was though for spellcasters and I think it should stay like that.
Ask yourself if it would be as disruptive as time stop of wish.
If the answer is no, then just scale the level appropriately. E.g., would the wizard rather memorize this spell, or overland flight? How about dimension door? Etc.
Few ways to accomplish putting mirror image on others.
Marionette Possession and cast it on them.
Ring of Spell Storing
Ioun stone with spell storing
Brown Fur Transmuter Arcanist
Here's my advice, Displacement is in generally a better effect. Sure, Mirror Image can have a higher miss chance, but it steadily decreases with every attack. Make a "Greater Mirror Image Spell" which has a target of creature touched as a 3rd level spell instead of personal. Maybe increase the maximum number of images to 12 (maybe 1d6 + 1 image per 3 caster levels). Duration...either keep it minutes/level of rounds per level. I'd probably do rounds per level.
This should be relatively balanced.
