One possible option for adding more summoning spells is to do a series of quests where the quest rewards is a summoning spell. I've not the time to implement all the details of the quest and the code required, otherwise I would do such a thing.
My summoner mod was my first attempt at making summoning a little more viable for the player who wants to summon creatures. Namely it adds units with the ability to summon an elementals. Unfortunately, this is horribly unbalanced because I was unsuccessful in getting the AI to use them effectively (difficult to test), and so my compensation to this was to attach these units to the books that have a 1 and 4 chance of showing up on your research tree.
You can pick up these characters from a quest that you do a special breed of summoners. In the code I have also altered the summon spell that these units use. They can only cast their summons in tatical combat, but it costs you no mana to do so. I still have the limit in place, but it is per summoner group so in effect you can have 3 fire summoners bring in 3 fire elementals. Then you effectively got yourself 3 mages and 3 fire elementals that you can summon back when the fire elementals die.
Also, there is much difficulty in tying the ability to the summoner profession that one chooses from the beginning. I have tried to make it a pre-requisite before and was unsuccessful, but doesn't mean that the code hasn't changed enough to allow one to do this now.