The 6.1 PTR notes went up on Friday (Wowhead has all the details here), including the new follower missions that will be implemented for the Blackrock Foundry raid. Here are the four new missions and groups of threats you'll want to be able to counter!
Slagworks (Versus Furies)
Danger Zones, Deadly Minions, Group Damage, Minion Swarms, Timed Battle, Wild Aggression
Iron Assembly (Underground/Caves)
Danger Zones, Magic Debuff x2, Minion Swarms x2, Powerful Spell
Black Forge (Mountains)
Danger Zones, Group Damage x2, Massive Strike, Timed Battle, Wild Aggression
Blackhand's Crucible (Versus Orcs)
Danger Zones, Deadly Minions, Group Damage, Massive Strike, Timed Battle, Wild Aggression.
Of these, Black Forge is the one I'm most concerned with. Double Group Damage is a difficult obstacle to overcome, especially when you consider the other four threats in that mission. There's not many follower classes and specs that even get access to Group Damage as an ability, let alone ones that can have Group Damage and one of the other threats.
But 6.1 is still a ways off, so we have time to recruit and adjust followers accordingly. Here are the only class/spec combinations that can have Group Damage and one of the other Black Forge threat counters.
Druid, Restoration - Timed Battle* (maybe)
Monk, Brewmaster - Danger Zones, Massive Strike, Wild Aggression
Monk, Mistweaver - Danger Zones, Timed Battle* (maybe)
Paladin, Holy - Timed Battle* (maybe)
Priest, Discipline - Danger Zones, Timed Battle* (maybe)
Priest, Holy - Danger Zones, Timed Battle* (maybe)
Shaman, Elemental - Timed Battle
Shaman, Enhancement - Danger Zones; Timed Battle
Shaman, Restoration - Timed Battle* (maybe)
Warlock, Destruction - Timed Battle
(Special) Soulbinder Tuulani - Danger Zones; Timed Battle
(Tuulani is a Shadow Priest, but comes with Prayer of Healing, a Group Damage counter, which is normally not available to follower Shadow Priests.)
(*I'm not sure if I missed these, or if additional threat counters were added to Wowhead's lists since posting, but there's apparently more workable combinations than I originally thought! Every healing spec can apparently counter Timed Battle, so I've updated the list accordingly. However, these counters all use now-removed mana abilities like Innervate or Divine Plea, so I'm not sure if you can actually GET these abilities on followers...)
Now granted, you can always (and probably will) use secondary trait bonuses to increase your chance of success. (Wowhead has an excellent guide on increasing mission success.) But if you're like me and want to aim for perfect 6-for-6 threat counters, you'll want to headhunt or level up one of the above follower types and hope you get you lucky.
Besides Tuulani, there are also a number of other followers that can be recruited during questing or purchased from vendors that can end up with one of the above ability combinations.
NOTE: You might notice some followers who are one of the above specs missing from the following list. Quest/purchased followers sometimes have different options for threat counters (like Tuulani). For example, Leorajh is a Resto Shaman, but if you look at his list of potential counters, he cannot get Water Shield, the Resto Druid counter for Timed Battle.
Horde players can recruit Lokra (Enhance Shaman) OR Kal'gor the Honorable (Ele Shaman), Kaz the Shrieker (Ele Shaman), and purchase Professor Felblast (Destro Warlock, revered with Steamwheedle Preservation Society).
Alliance players can recruit Rulkan (Enhance Shaman), Fiona (Holy Priest), and purchase Professor Felblast (Destro Warlock, revered with Steamwheedle Preservation Society), and Cleric Maluuf (Holy Priest, Revered with Council of Exarchs)