[DraconisRex]

You're kind of in a bind. Many classes will be useless to you.

Ninja. Sorry, too late. By the time your new Ninja would even approach the faintest glimmer of a chance, the game will be over. Monk. Ditto. Gadgeteer & Bard will be hopelessly limited, too.

Rogue wouldn't be much use to your party. You can't fight, and who cares about the rest of the abilities by this point.

Fighter, you're probably way too weak. Maybe a polearm fighter if you've got the stats. Valykrie, only if gender and abilities would allow. The same with Lord, with the additional problem that Lords develop LATE and you'll never make it to their peak performance.

Ranger, depends on your senses and level up choices. You can be "ranged" with throwing & sling, too. At least you'll have more offensive fire-power and better armour.

Bishop & Priest are both workable. Bishop will get you three more schools. Make some money and you'll fill in a majority of your missing spells quickly. Priest will give additional healing to supplement your offensive magic, but I doubt that's really needed.

Alchemist would get you better armour. But at those levels, that's not really an over-riding concern. You'd also add a few spells that are semi-duplicates of Mage spells. Plus "Heal Wounds" and a few Priest duplicates.

Psionicist would be an excellent choice. Many creatures don't have resistance to the Mental Realm. Ego whip is a kick-butt spell. You should have a lot of points in the Mental Realm to help with spell choices. Plus there are non-combat spells to help you develop your skills quickly. And they over-lap in equipment very well, so you probably won't suddenly find yourself without something important.

I'd go with a Psionicist. Use detect secrets & other mental (Psionic Realm) spells as much as possible. In and out of combat. All the time.