Oh crap. Your party is druid, worthless, and situationally worthless, and you're thinking of plugging more worthless into it. Cut the poor druid some slack, he's got enough on his plate trying to do the job of three people while keeping the two who aren't pulling their weight from getting themselves killed.
Your "tank" is going to fail and die. Whether or not you accept the legitimacy of the role, fighter can't do it. You'd be better off adding a cleric if you wanted to stand on the front line and soak up punishment. Cleric can actually recover health when he loses it, can protect himself from losing it with certain defensive buffs, has better base saves, and is a better attacker when you account for things like the Divine Power spell.
It looks like what your party really could use, though, is a wizard... specifically a specialist transmuter. Buff up the monk -- he's no good on his own, but if he's enlarged, flying, and mage armored he'll be... slightly less crummy. Paralyze or blind enemies, or force them to balance precariously -- the rogue will appreciate getting more sneak attack opportunities. Haste your homeboys and slow your foes. And remember that you can use schools of magic other than your specialist school.
If you do opt to go with an arcanist, stay away from the sorcerer. It's too easy to screw up because the choices are mostly final. With a wizard, at least if you pick the wrong spells to know, you can learn more with money for scrolls and time to write in your book.
