The lack of lore on some of these Titans makes it pretty difficult to give them spells/abilities (like Kargoth for example). I've got no idea if there was more lore on these guys in previous games, but any info that can be brought to light would be useful. This is my take on Titans:
Antharg - Horseman of Plague. This one's one of the easy ones; stuff like Rot and Chaos Plague. Maybe even Transform or Mutate since he also does some of that stuff too.
Balora - I've not got clue who Balora is or what she represents. I can only guess since she represents a giant Succubus that she's inclined towards seducing or warping the minds of her enemies. I'd probably go for something like Corruption (probably at a reasonably high level) and some kind of new spell (which attempts to convert enemy units to her side) or failing that, Awe.
Bane - Another easy one, Breath of Dying and Call the Dead.
Gornak Elfeater - Half troll, half orc creature which led the Orkish Jihad. He was apparently resurrected at some point as an undead creature. Despite an increased power level after death I don't see how he can compare to some of the other titans. But anyway, he should probably have that Call the Horde ability Zeto mentioned and a large command radius.
Grond - I guess Grond is just a giant golem. If it is, it's another titan with a pretty low power level really. Grond appears to have no abilities besides hitting things really hard, so I'm stumped here.
Iriki - Apparently the last of an ancient race of Ssrathi (I wonder if he has anything to do with the dragons?). I assume he'd have various abilities relating to the sun, so maybe Pillar of Fire/Armageddon and Light/Darkness (maybe the weather should become clear and sunny and stay that way for as long as Iriki remains on the battlefield?).
Ironbark - A really big treant. I assume it's the oldest living treant or maybe the first treant. I must be missing some lore here
. Can't see him doing more than a high level Entangle and an aoe Life spell. Probably not a titan that's particularly high on the power chart really. I can't see Ironbark winning in a battle against Bane or Sirian for example (in lore, not in game), but I really can't say since I know so little about many of these titans.
Kargoth - Essentially a dark elven daemon god of magic. I think something like Call of Kargoth and Call Lightning/Armageddon/Wildfire (or some ability based on how many things the player or Kargoth has killed).
King Lunarion - Lots of lore on this titan, but I'm not sure why he isn't called The Moon King, unless this is actually the spirit of King Lunarion called back from the heavens to do battle again. In which case that would be pretty cool and I'd imagine he'd be pretty powerful too as titans go. Lunarion should probably have a command radius like Gornak since he's a leader figure and spells like Major Healing.
King Khalid - Carries the Hammer of Morannin? Is that the hammer which was used by the Dwarf god to create his race? I can only guess that Call Lightning, Summon Stone Golem or Summon Bronze Golem would be suitable (more like crafting than summoning in this case).
Melkor - Probably should have the same spell as his Dunekeep, but stronger and effecting more mines? Maybe the weather could turn into a sandstorm at his presence. Could also potentially have a really long range Drain Mana I suppose if that counts as a famine or sorts.
Sirian - God of the Sirians, he should probably have stuff like Purify (or maybe a version of Purify that can hit good and evil?).
Skull of Sartek - Really high level Berserker, Vampirism, some kind of Morph spell maybe.
Tempest - Call Lightning and Storm should work.
The Forestmaster - I'm guessing there were many Forestmasters at some point and that this is the last one. Still haven't got much idea about their powers. I suppose summon Sprite and a spell that summons something defensive would work.
The Lion Throne - I can't find a name for this guy anywhere, only that he's liked by his people. Bit strange considering he sounds like a maniacal warmonger
. I still don't know how the Empire is considered good... anyway, no idea who this guy is, what he does or even how he's able to do anything except sit in a chair and give orders.