I'm thinking (assuming /follow is in, which it should be since it IS classic):

Spriest+4afflocks - dots everywhere in PVP, lifetap for mana, VE for health replenishment. Spriest mana is an issue. Dungeons would be hard.
5 shaman - mobile artillery in PVP. No reliable cc. No dungeons.
4 shaman+boomkin - mobile artillery in PVP. Respec your boomkin and 1 shaman for dungeons and you're fine for 5's.
priest+4mages - big single target and big aoe in PVP. Gimmicky but doable 5mans (lots of cc) for dungeons (Xzin did it anyway).
or a rogue/druid combo If I don't want to 5 box - leave the druid in the bushes to heal/dps if your rogue gets in trouble. double stealth everywhere for open world pvp goodness.

IMO It's hard to argue with the aoe firepower of priest+4mages for BG's, especially with mouse sharing being a thing now where it wasn't in original classic (5 blizzards dropped on their bunched up team in AV anyone?). The shaman are a little more durable, but I'd bring the druid instead of a fifth shaman to give me some flexibility for dungeons. I don't love shaman when melee gets on top of you though versus aoe fear or frost nova/blink of priest+mages or the auto tracking drain spells of locks/spriest. The healing of the spriest/lock group is the weakest, but they are also all pretty beefy HP wise compared to mages. And dotting the hell out of everyone in BG's is pretty entertaining (and redundant...I did it with 5 Spriests in BC).

Still a lot to think about and a long time to think as well. I do expect /follow to work in Classic even if it's only a FU from Blizzard saying "No no no, you told us you wanted CLASSIC. Well here it is, just like it was in 1.12! Suck it up and learn to deal with multiboxers!".