As one who enjoys summoners, I figured I'd drop my .02. It's late, I haven't slept well, and this is mostly on the fly, so hopefully it flows well and makes sense.
Let's start with the good. The starting Warg for the Summoner Trait is pretty good, and about right for the cost. The Familiar can be quite useful, and will need to be taken into consideration when making changes to summons. There is a good variety of summons, which is always good.
Now for the iffy part, the elementals. I like the Elemental summons, in concept, but in reality they could use some help. The +1/+2 to summons can only do so much, and forcing the player to get them in a specific order does not help the player who lacks certain elemental nodes. Instead, I would suggest one of three things:
1) Summon Elemental – Replace the four summons with a single summon that then allows the player which element they wish to summon. The biggest drawback to this is that there is no mechanic in the game to support this. I guess you could use the party screen (where you choose the character to cast a buff on) and replace the party with each of the four elementals.
2) Unlock all four elemental summons at the same time, allowing the player to choose which elemental they wish to summon. This would be much easier to implement, would allow for more summons overall (3 more to be precise), and would allow summoners to focus on the element they have the nodes for.
3) Split the Path of the Mage and create a Path of the Summoner. This would allow the Mage to have more casty spells, and allow for a full-on summoner path. This, however, would require quite a bit of work and may be outside the scope of the current project/schedule.
Of the three choices, I prefer the third, followed by the second, with the first one being the least impressive. The main reason I like the third is that it grants a second mage tree. I have to admit that I find it odd that there is only one magic-focused path, and three melee focused paths (and one commander), in a game focused on magic. 
Now, as for the summons themselves, if you go with #1 or #2, I would have them scaled to the level of the summoner, and limited in quantity to the number of elemental nodes they currently control, plus one. Additional elemental summons beyond the first have an incrementally increased mana cost and upkeep (eg: +15% per add'l summon). Then balance the elementals and allow them to gain new abilities as the summoner reaches higher levels (eg: Ice Elemental slows on hit from 1-4, at L5 they gain the Slow spell with a 3 turn recharge, 2 turn at L6, 1 turn at L7, and can cast every turn at L8, then they gain a new ability at L9, etc).
#1 and #2 would also allow you to add in more types of summons, to complement the Ignys and Crags you already have. It's a shame you don't have any Mephits, as those could be a good low level strategic elemental summon.
For #3, you could have the first page focus on elemental summons, as opposed to the elemental schools, and the second page could focus on non-elemental summons and overall summoner buffs (lower maint cost). For the elemental trees, you could use the Shrills, you could modify the Assassin Demons into Mephits of sorts (alter texture, give them an aura, etc), have both lesser and greater elementals, rock spiders, ice warg, ignys, etc. Non-elemental summons could use naja, skath, hergons, etc. If I went this route, then I would probably put a debuff of sorts (maybe -x levels?) on summons that are not in the same army as the summoner (with some high level traits to lessen that debuff, allowing late game summoners to continue to grow).
Elemental Forums
Stardock Forums
Hard to max out due to slower leveling.