DK and monks are both good picks I think.

DK's always a strong pick because of the self healing and with the return of AMZ you have a good party wide magic defensive + he gives a battle rez (which is important to consider if you don't pick druid healer or run boomkins).

I do see Monk pulling ahead for a hunter trinity comp, especially if you run a druid healer (because of the brez). Because of the way monks stagger damage and the fact that you have 3 spirit beast heals (+ potentially druid resto affinity passive) that can consistently reduce the stagger on your tank (or burst if you combine them), you will have very consistent tank damage. Another bonus for the Monk is the 5% physical damage debuff he brings which if you run hunters, is a straight 5% damage increase for each hunter. I also see it as a bonus if you run a druid with a monk is that they share the same armor so you can easier trade gear between them and put the gear where it is most useful (within the boundaries of personal loot rules of course). Of course Monk is a slight gamble because he is currently broken and they might just nerfhammer him into the ground.

Personally like DH tanks but they have too many gaps in defensive to be stable (which if you run 1 character is fine, but not for multiboxing).
Paladins are strong picks with dps that exceeds some dps classes but they have a few changes that I dislike (like the nerf to their big heal).

Healer wise shaman becomes an extremely strong pick if you choose not to go the hunter route with the nerf to bloodlust scrolls. And since as a multiboxer you don't have people scattered too much you won't lose out on the weakness of shamans. This becomes especially true if you have secondary healing capabilities on your tank and DPS to offset potential weak single target healing.

Druids are a good pick and if you can get the HOTS rolling (which someone made a topic about on these forums not too long ago explaining how) then you are golden. Only questionable part here is mana usage (is that still a thing?)

I am really looking forward to Shadowlands. With all the changes going on I might step away from 5x Druid and go BrewM/Hunters/RDruid (I always main a Druid), but time will tell.