If you want to do some PvE, then include at least one class that can spec Tank and have a different character that could spec Heals.
That will allow you to use the Random Dungeon Finder.

For example:
- DK + 4x Rets.
- Pally + 4x DKs.
- 5x Druid.
- 5x Paladin
- 4x Shaman + 1x Boomkin

What are your goals, as far as PvP goes?
Enhance can essentially auto kill, almost anything, once every few minutes... making them likely stronger for arena.
A DK heavy team, doesn't have the instant burst to kill anything before they can react, but have the tools to be very dominant in a battleground environment.

I'd probably go with a 5x combination, with Pallies and DKs.
Likely, 1x Pally & 4x DKs.
But mix them as you will.