Go with a combo you think will be fun.

Any DPS with any healer will be a viable combo.

My first ever main was a lock.

In BG's as an affliction lock I was pretty hard to kill (apart from rogues).
When I was lucky enough to find a healer running with me I was able to defend graveyards against far superior numbers.

This was pre wotlk and pre patch 3. A long time before.

There wasn't the burst damage that we have now, but I guarantee you would still have fun.

Incidentally I would run the warlock as the main and use the pally as a pocket healer.
Warlocks are one of the easiest classes to solo - with a healer and lifetap your down time will be minimal.
Macro up healing the warlock, and healing the pet so you dont waste time having to click on the voidy/felguard.

If you want any macros for warlocks - or advanced ways of playing definitley go to wowmb.net (the warlocks den).
I played to level 45 on my lock before discovering the warlocks den. Then realised I had no clue about how to play my class properly.
(for example I never ever lifetapped - I actually thought - "If the mob is killing me, why would I want to help him by reducing my health. I also never used a soulstone as I didn't see the point when I could just run back from a graveyard. DOH!!!!)

As to levelling a druid. Do it. You won't regret it. Especially if you are RAFing.
Druids are dull until 20. Most other classes you know you wil enjoy playing the class long before you hit 20.
Druids you can't decide until well after 20.

My only regret about levelling the druid is that I didn't do it sooner. (And I'm RAFing up a second one, so I can have one feral cat and one boomkin).