My other ramblings:
Further to the whole mage thing though (which might require it's own thread). I think part of the reason mages aren't used is that their ability to soften defenders has been duplicated in potentially more effective units, such as catapults and archers. On top of this, their summons are highly dependant upon factors having nothing to do with leveling the unit, and if they happen to spend another 3 points into a tree with summons they don't gain any benefit from it (there aren't really enough weaknesses/strengths vs particular elements to justify summoning something different). Balancing this by making the mage gain extra combat prowess just makes them into another direct attack unit that I would argue removes their unique role. What I would suggest doing though, is making summons permanent, and adjusting the mage units to function like commanders (each 3 ranks in a spell tree can let them maintain one more unit), that have their own set of promotions to assist these summons. In the alternative of focusing on summons, mages should be able to select promotions that increase maximum damage of spells, range of spells, and even targeting functions for them (casting lighting on a tile 3 squares away for example). Also.. lower rank spells should scale with higher Ranked casters. So a skeleton cast by a lich might have 10 Str and guardsman instead of 3 Str when cast by an adept. Further.. and one of the biggest concerns.. is that mages should acquire xp from damaging or killing opponents with either thier summons or their spells, so they don't fall behind their 200 xp Mithril Phalanx counterparts. Oh.. and tier 3 creation spell.. clone anyone? ^^;
I like this idea for making summons permanent, that way when a summon gets experence it doesn't feel like a waste of time selecting the promotions. The summoning trait could also add 1 to the amount of summons a mage can hold.
Other thoughs
Summoning Spells could now take longer to cast (2 or 3 turns) rather than a *poof*, oh look, a balor
To make summons slightly more unique, you could allow the location of the casting to affect the summon.
Examples
Balor: may sacrifice 1 population in a city with a sacrificial alter to give it Burning Blood
(Killing an enemy prevents it from turning barbarian for 5 turns, as does sacrificing a unit, it starts 'sated')
Fire Elemental: When summoned in the Pyre of the Seraphic. they do two extra fire damage. (Or increase collateral damage)
Earth Elemental: When summoned in the Standing Stones they gain the teleportation ability. (Tunneling through the ground)
Spectres/Wraith: When summoned in the Broken Sepulcher the unit starts with cannabalize (Or a new promotions called Terror, has a chance to bypass courage)
Air Elemental: When summoned in the Maelstrom the unit starts with Air II (Thus able to cast the spell Maelstrom)
Water Elemental: When summoned on Aifon Isle, when the unit splits they will start at full health and have half the experience of the main unit. (Perhaps also have a mini water elemental have a chance to turn into a proper elemental after a successful victory, but these won't be empowered by the Aifon Isle)
Ice Elemental: When summoned on Letum Frigus the unit can create a blizzard
A new promotion for summons could be added requiring combat 5 allowing them to free themself from their master allowing more summoned units (perhaps similar to the way freelancer does it)