Melee + Caster works just fine, or even great for PvE.

If by PvP, you mean "Battlegrounds", you can also be somewhat OK with melee+caster. Druid as a heal/buff bot for a rogue is interesting, probably would be some fun in BGs.

Arena... no way, unless you plan to just run a sub-1400 rating to get welfare epics (the few that can be had for no rating requirement, that is)