I'm not sure if your asking about pve or Pvp but I'm making a mixed team for pve content.

i have just finished leveling them to 90 it's a blood dk, resto Druid, BM hunter, warlock and a shaman.
the reason I chose these 5 is that they have very little gear overlaps and also all but the shaman have a battle Rez and he has reincarnate,
i feel this will be useful when for hard boss fights and whatnot

as for plans in WoD I'll be leveling however I feel is easiest (questing or instancing) then I'm going to get stuck into 5 man content
what professions will you be leveling first and is it to benefit your teams gear or gold income?