Kensai -lvl13> druid IS great, as the Avenger -lvl12> Fighter is.
Nice Options also Cleric -lvl12> mage, thatīs the mega-Aerie. However difficult to solo, I admit.
Druid-Thief combos are interesting though impossible, sorry.
Ever thought about dualling OUT OF A specialist mage? There are interesting, though mostly difficult options, too, however, mage -lvl11 or lvl 13> Fighter is something interesting to consider...isnt it...
Avenger -> Fighter would be horrible, and its not even possible unless you increase your strength first since they have a -2 penalty at creation. Dualing from a mage would be pretty bad as well. The thing with fighter -> duals is the massive stat requirements (pretty hard for a human fighter with low minimums) which means you will probably have to sacrifice something since druids have two primary requisites.
A priest of Lathander dual to something would be an interesting option due to the extra attack you get from the Boon.