I have a level 14 Projectinoist mesmerist who is about to hit 15.
He specializes in enchantment and possessing both creatures and objects. The obvious dip is Crossblooded Sorcerer, to shore up weak points against immune creatures (or bolster compulsion DCs, but that's probably overkill). Perhaps there's more flavorful options out there — ones that are at least somewhat optimized? For example, swashbuckler might be a fun way to leverage very high charisma while possessing things.
Any other ideas?
2 levels of Paladin if you fit the Alignment
Lunar/Nature/Lore Oracle for Cha to AC
Maybe 2 levels of Ninja for the Swift action Invisibility or Disguise Self.
Honestly Swashbuckler is pretty solid for the deeds, although you can pick Dodging Panache up with a feat.
There's a class with rage to make for better smashing while possessing things. Although getting to L16 mesmerist for greater object possession would likely help there. The mad magic feat and a bloodrager could work though.
The chevalier PrC is like a paladin dip with a looser alignment requirement.
Various prestige classes are aligned to some deity or another. Is your mesmerist religious?
Nope, he’s not religious whatsoever and neutral aligned. So, paladins and many other alignment or religious-based options are out.
Seducer witch might be worth looking at.
If the character has a decent Int, then psychometrist vigilante could expand on "possession" of objects. Also, Dual Identity and Seamless Guise are pretty nice.
Ninja (2nd level for Cha-based ki-pool and access to ninja tricks) can improve skills a bit and add sneak attack.
If you want to improve combat-effectiveness, id rager bloodrager might be interesting.