I’ll assess all of the specs you’ve listed.
Warlock
Fun in TBC because they’re cracked. Great choice. Great arena viability as well.
Feral/Resto/Moonkin
Feral tank is cracked. Feral cat actually has a chance at getting invited to raids. Resto druid is great the entire expansion and has great arena viability, especially in later seasons. Moonkin also gains the ability to get a raid invite which is a huge breath of fresh air for them.
Rogue
Gods of arena, and gods of not getting invited to raids. Personally my favorite bank alt. I level it to 70 and level lockpicking, jewelcrafting, and alchemy and go to town. IMO not a great choice unless it’s a PVP focused alt.
Enhance
Another class that gains a purpose in TBC, although it does lose its two-handed identity and just turns into another dual wielding melee DPS. Not a ton of arena viability. Okayish choice, but I’m bias because I don’t like them. Nearly guaranteed raid invite.
Shadow priest
THE spec if you want to make friends out of casters in your raid. I don’t know about arena viability, but if you build disc spec for PVP, it’s top. Imo one of the coolest specs from an RP stand point.
Fury/prot war
Fury is a social spec, as in you better be loved in your guild or you’re not getting the one fury spot in the raid. Switch to arms improves your chance of getting an invite by 0.2373%, again amplified by how many people in your guild like you.
Prot war is not as high performing as the other two tanks in TBC, but is by no means incapable of functioning as a tank throughout all the content. My casual guild killed KJ with a prot war main tank.
Prot paladin
Gods of AOE tanking, demigods of farming gold, will probably be competitive because at this point I think half of everyone I’ve talked to plans on making a prot paladin alt.
Ret paladin
Not terribly easy to get a raid spot. They’re needed, but it’s you vs a million rets. I don’t know their viability in PVP.
Your locked in characters:
Warlock - Caster
Choose one:
Feral - Melee
Resto - Healer
Moonkin - Caster
You could choose based off of archetype variety:
Position 1: Caster is locked in warlock, which eliminates shadow priest and moonkin
Position 2: Melee vs tank vs heal druid
Position 3: Melee vs tank vs heal, excluding the choice made for druid at position 2
Of course noticing that if you don’t choose resto druid for position 2, you have no other healing option unless you change your mind about the selection allowance for the third alt.
Another consideration is likelihood of your alt being invited to a raid:
Enhance shaman > shadow priest > and the rest seem equal in chances unless your guild loves you.