Thanks for the replies. I will definitely post a minimod on the mod section and put link here. Just need to fix some more numbers here and there.
There are some very interesting and right suggestions, but probably due to the fact that there are either bugs and/or limitation, they are not possible to implement as far as I know.
For instance, ^2 for each summon and limiting the same summon to certain number other than 1..... as far as I know, they are not possible to implement (unless Heavenfall says otherwise.)
To fix the beastlord's issue I tried to implement variable mana cost system, but then both Heavenfall and I found out it is bugged that taming (or whatever any other using that specific call) does not work with variable mana coding.
That said, in my mini-mod, the mana unkeep is varied based on summoned's monster's strength. For instance, Death Demon / Shrill Lord will cost 250 / 400 mana to summon and needs 14 / 20 mana unkeep (no worries they are buffed enough to match such high requirement) and they will need research (Third Book of Magi) to unlock.
That said, my mod indeed undermines early-game troops to a bit.... but then, nothing is more game breaking than late game troops, and from my game, I haven't seen my cities ever stop building things until the game ends because everything takes too much time to build in the first place.
It was terrible idea in the first place to merge build and train quene, and should had been separated like original Elemental anyway.