Shaman Warden Tank
This has to be the most popular class/spec combination that players would want to see come back in Classic+. Shaman's original talent tree has a lot of tank/mitigation talents when you spec into the Enhancement tree, like Shield Specialization, Anticipation, and Toughness. They would also have abilities like Stoneclaw Totem and Earth Shock that allow them to generate high threat. At lower levels, they also can actually be very viable tanks in leveling dungeons. Despite all that, however, Shaman never got gear in their armor type - being both Leather and Mail - that really allowed them to achieve that role to its highest potential.
Season of Discovery would give the Shaman new runes such as Spirit of the Alpha, Shield Mastery, Overcharged, and Way of Earth, which would allow the Shaman to be an actual shaman tank with these new runes engraved as abilities. Once we got to Phase 4, Shaman tanks would also get tier sets that continued to strengthen this class/spec combination.
Since a lot of the talents currently exist in the Vanilla talent tree, it could easily be something that could be implemented once again in Classic+, and arguably the most in-demand build to make a return in another Classic game.
Melee Survival Hunter
Melee Hunter became extremely popular when it was introduced in Season of Discovery. Although you could say that the Melee Hunters were secretly Fury Warrior mains hidden in disguise, it was still a very popular class/spec combination in Season of Discovery. Runes like Dual Wield Specialization, Flanking Strike, and Melee Specialist made this possible, with content creators like Sarthe very much hyping this class and spec!
The Burning Crusade sees Hunters practicing a rotation involving "melee weaving"; perhaps the Melee Hunter in Classic + could do the exact opposite, weaving in ranged abilities while their primary abilities are melee! Although maybe it shouldn't gap the rest of the DPS classes like it did during Phase 2 of Season of Discovery.
Protection Paladin Tank
This one's weird because, technically, yes, Protection Paladins do exist in Vanilla. However, it isn't well known that they lack the abilities, spell coefficients, and gear to actually truly be a viable tank in raid content. The Vanilla Protection Paladin lacks a taunt and misses out on any true gear and set bonuses that push the Paladin Tank to actually gain power and just perform in a raid. Tanks tend to tailor themselves around their team, but a team with a Protection Paladin would need to tailor themselves around the tank, which is counterintuitive.
Finally...
The Protection Paladin in SoD was built more or less around the Wrath of the Lich King Protection Paladin, with iconic abilities like Hammer of the Righteous, Shield of Righteousness, and Avenger's Shield. The rune Malleable Protection would also mimic both Touched by the Light and Divine Protection, again even strengthening the Wrath-like Protection Paladin build.
However, would this be the ideal build for the Paladin Tank? There is an argument for players to yearn for more of a TBC-like Protection Paladin, pushing more towards Spell Power and focusing on consecration, rather than focusing on Attack Power and the Wrath-born abilities mentioned above. What would you prefer? Let us know in the comments!
The Controversial Oddballs
These next few we want to group together because these are truly the "oddballs" that would stir quite a controversy if added. We, of course, are talking about the Arcane Mage Healer, Warlock Tank, and Rogue Tank. It's hard to tell if players truly want these classes to return based on the argument of what the "spirit" of Classic is.
The intent of these new class/spec/builds was to have more tanks and healers available through popular classes people already played. However, is that the direction players want in a Classic Plus game? Blizzard has shown through trial and error that all of these unique class/spec/builds can work, especially Rogue tank, as it was one of the best tank classes in SoD, but is this the direction most players are interested in in a game outside of SoD? We can't definitively answer this, but we'd love to know your thoughts in the comments!
