I was recently counting the mages from the different schools and noticed a few things. The most obvious is that 3 of the 4 core set mages have an alternate of the complementary gender appearing in an expansion, with a female Wizard noticeably absent. Also noticeable is that the schools haven't been equally featured. So far, there are 3 Nature (2 Beastmasters and a Druid), 3 Dark (2 Warlocks and a Necromancer), 2 War (2 Warlords), 2 Holy (2 Priests), 1 Mind (Forcemaster) and 1 Arcane (Wizard). If you look at the Elemental schools (ignoring the Wizard since he can be any of these), there are currently 2 Fire (Warlocks), 2 Earth (Warlords), 1 Water (Druid, but low level only) and no Air (although the Wizard's recommended starter book is Air).
If we assume the Siren will be Mind/Water and the Paladin will be Holy/War (? - could it be the first to be trained in 2 non-element schools?), then we would have 3 Nature, 3 Holy, 3 War, 3 Dark, 2 Mind and still just 1 Arcane. The elements would be mostly evened up, with 2 Fire, 2 Earth, 2 Water (although still only 1 that goes beyond low-level spells), but still no Air.
In order to even things up, it seems to me that the most obvious expansion after Paladin/Siren would be (1) an alternate female Wizard who specializes in Air and/or Water (with a preference for ice magic), and (2) an alternate male Forcemaster. The former would finally give us a second Wizard option, although it might be hard to balance without making the existing Wizard even more powerful (e.g. strong water/ice and air/wind spells); the latter would give the Forcemaster some help (as was done for the Warlord), since she has become one of the harder mages to win with.
Alt Wizard evidence: See recent promos including Windstorm, Gale Force Ring, Rain Cloud, Acid Blast, Staff of Storms, Lightning Strike, Wall of Poison Gas, Lightning Beetle, Leviathan Scale Armor, Storm Drake Hide. Some of these feature art with a white-haired female wizard (see Acid Blast and Windstorm), also shown here:
Of course, several of these promos, especially the Water spells, might be included in the Paladin/Siren expansion. Further evidence includes the Frost and Defrost keywords in print but without a purpose (assuming they aren't Siren-y), and I'm sure I'm not the only person who would like to see that rectified. And like others, I'd love to see some ice-related terrain.
Forcemaster evidence: Not so much, aside from the Forcemaster being in need of help and currently alone in her play style (aggressive and largely creatureless). There's the Wall of Force promo, but not much else. More mind spells would certainly be welcome, especially more illusion-like creatures, more Mind conjurations that help deal with the board state (analogous to Harshforge Monolith, etc), maybe a new spawnpoint/familiar or two (e.g. an analogue to a Battle Forge or Wizard's Tower for Mind mages), new equipment (Force Armor?), etc.
