(Small thing, I started writing this back in mk2 then when mk3 was released put it to one side...)
The Stormblade Captain is a great example of this, before it was released the only "Storm knight solo" was Laddermore and there wasn't a "Stormblade solo". Both of these are types of solos allowed in various tier lists.
Stormsmith units are another example from Nemo & Styker's theme lists in mk2. The only Stormsmith unit was the Storm tower light artillery. As with this example there were tier lists that allowed Stormsmith units and others that only allowed Storm towers, so that showed they would release another stormsmith unit which they did in mk3 with the Stormsmith Grenadiers.
Ranger solos - Kraye & Magnus mk2 themes
No models that fit this yet but I think it would be a good way to add a solo that hands out pathfinder in faction. If it is not a character that would be even better!
Light cav solos - Kraye mk2 theme
Currently another empty choice, not sure how this could be used or what role a light cav solo could have in Cygnar. (The light cav rule doesn't even exist anymore!) Perhaps a Character Tempest Blazer that buffs them much like Laddermore does for Storm Lances?
Morrowan units - Blaize mk2 theme
Other tier lists name Precursor Knights individually so it could be assumed that there will be another unit at some point. Hopefully a multi wound medium base unit, or maybe a cavalry unit?
Morrowan solos - Blaize mk2 theme
Currently only Harlan Versh fits in this category it would be good to see a Morrowan priest/cleric style solo that could prevent friendly model's souls going to enemy models in it's command area. It could also have a medic aura and grant tough as a Action.
Sword Knight solo - Magnus mk2 theme
I'm not sure what could be done with a Sword knight solo, my thoughts first went to Runewood but he is not typed as a sword knight. There are not many buffs that I can think of that Sword knights could be given that don't already exist in the faction. This makes me think a combat solo might be likely in that role.
It would be cool to see a "Sword knight veteran" With jack marshal and a rule allowing it to bring Talon merc jacks into a cygnar army making them Friendly faction so Sword knights can use them to enable Flank.
The Storm division theme doesn't offer any real hints for new models sadly, but this is a theme within Cygnar that is really fleshed out so its not overly surprising.
The Heavy metal theme. Notice how it mentions "Sword knight models/units". Another link to the clue above that there might be a Sword Knight solo in the future or perhaps even another unit?
Sons of the Tempest them has a small clue to give, it states "small based Arcane Tempest solo" which hints towards a medium or large based one in the future. This could be the light cav solo mentioned above, so perhaps a character Blazer solo or Dragoon.
What do you think the future releases could be?
I'd love to know your thoughts on the subject.
(a lightning question mark to inspire...)