I find the summoner spec... boring. I try to love it because it's my playstyle but I find it not enjoyable to level my sov.
Only 1 summon / spell, why? There is a maintenance cost so why restrict the number of summons? Increase the maintenance for best summon, but allow us to create army of summons! The drawback is that you only get 1 summon with the summoner perk, and you need to wait level 4 before unlocking a second one (mage tree) !
Linear progression. Only earth magic get some summons, you must take mage spec, and even here the progression is really linear : Ice ele > Air > Fire then a branching and more linear unlock. Combined with the 1 summon limit, progressing as a summoner isnt exciting. Add the +1 / +2 summons node, more useful for the tactical summon than the strategic one I find.
I think fire, air, water, life & death magic perk need 1 tactical summon & 1 strategic summon. More branching for summons in magic tree, so you can unlock the summons you need for complet you army and each game can be different. I would prefer less summons in the mage tree, so you could buy some other magic (Fire, air etc... with some summons inclued in the list).
With the linear progression, miss diversity: spec in necromancy (be able to raise dead troops as permanent undead troops on the strategic map, lich, momies, ghost etc...), elemental summoner (all the elemental creatures), beast master, demonologiste etc... I was a huge fan of master of magic, and I remember that in function of your spellbooks you got acces to different summons.
It's just my feeling after playing LH.