My feelings are in line w/ the other posters.
I think Crusader/RKV will blow the doors off the party as far as I can tell. I kind of feel that way about a DMM (Persist) Cleric as well, but ymmv. If you focus on spells that buff your melee ability, you're not really stepping on anyone's toes, so I guess there's that. I might suggest a DMM (Quicken) Cleric instead, since they are great healers/support casters, free up your actions for melee, and are a little weaker.
I also like just the straight Crusader or even a Paladin type (especially if you can get something like Pathfinder's Paladin up and running). But, really, just a straightforward Crusader or Melee Cleric will fit the bill. As to the Crusader, people have tossed around Bardsader, but you only need a 1 level dip into Bard, and possibly not that if you use Harmonious Knight or something. Then you grab Song of the White Raven and maybe Dragonfire Inspiration and off you go. If the Bard isn't planning on taking something like Dragonfire, I don't think you could be stepping on his toes -- at most you're both doing party support, but that's a pretty general thing and you will have none of his skills, spells, diversity, and additional songs.
Another vote for Crusader might be some battlefield control -- Stand Still, Trip, etc. The party seems squishy, except maybe the Druid, so keeping the heat off of them might be really good.