Wow, this mod is a heck of a lot of fun.
A few thoughts regarding the Summoner tree:
1) I made a beeline for the Summon Drake trait, and I was able to get my first drake by Season 67. Not too much longer, I picked up my first Storm Dragon and immediately took on Vetrar easily around Season 80 (the Arctic boss that gives the amazing sword Heart of the Glacier along with other goodies.)
2) As a Summoner, it's possible to almost entirely ignore the Warfare research tree until later in the game, when one has developed a lot more research per turn. Likewise, Fortress cities are less necessary, and one can instead focus on Conclaves to boost mana (through use of Herbalist, etc. and the Meditation city enchantment) and research as high as possible. More mana = more summons = faster experience gains for the Sovereign, which in turn allows for more powerful summons as you go along.
3) Similar to #2, since military units aren't that important, metal and crystal are less necessary to develop. The buildings that are required to acquire these resources take up a long time in the building queue for cities for non-Summoner games. But as a Summoner, one can build research, mana, and growth related buildings significantly faster by being able to skip most iron ore and crystal sites.
4) A custom faction with Decalon and Scouting goes perfect with a Summoner sovereign (Mancers with 3 basic moves rather than 2 are particularly awesome for this.) Decalon allows for cheap access to all four elemental schools of magic, and it's also possible to spam Arcane Monoliths (outposts) with the amount of mana that's being generated from having so many Conclaves and other sources of mana generation. Often it pays off to place outposts on resources just as a means of denying them from one's opponents not to mention being able to develop a road network all across the map.
I realize not all people will necessarily min/max like I have here, but for the sake of testing the mod's balance I wanted to see how far I could push the limits. Keep in mind the game was played on a custom Medium sized map, with balanced options other than high world and high opponent difficulty. I set the number of opponents to 3 and made them all Empire so they'd tend to band together more often against me (granted, with my Faction Power score few would start wars with me -- after all, I have Storm Dragons!)
It would be great to see a way to maintain the depth that the mod brings to the game while keeping it in line with the usual level of difficulty. My first suggestion would be to add a moderate crystal requirement for some of the more powerful summons. Not that the crystal would actually be CONSUMED, but rather that it simply needs to be present, similar to the Summoning Pool. This would force players to mine at least some crystal resources. Metal could be used in lieu of mana for casting summons via a Transmuting spell. This would encourage players to mine metal and would keep things more balanced between Mana, Metal, and Crystal.
Secondly, the Warfare tree could add small bonuses to the summons all the way throughout. For instance, the Armor technology could give bonus defense, etc. This one would be pretty straight forward.
I'm just throwing these ideas out there for what they're worth. The mod is already great. Thanks for the hard work!