Well yes the balance is already close to perfection.
Perfect balance would be achieved when all spells and all units have a use in competitive play where all that amazing tech tree is worth getting and is effectively used in different situations.
You see, as the game is right now if you try playing with lust disabled you will notice that games can even become a bit more interesting and strategic as they do not reach that big power spike, we see more sappers more runes more dks and dragons many different options that are very much overshadowed by the power of lust otherwise.
Lust really is a fun spell and I would not want it to be weaker because it really was designed to be strong, but I would like it if it required a bit more brain power and planning for when to use it because right now ogres can be always lusted up and it offers no room for counter-play and it is devastating a lusted army will always be lusted there is no stopping no down time no drawback.
And that is why I feel we could while keeping lust a very strong spell make it a little bit less of a no-brainer, so that we get a bit of all of these other strategies back and more windows of opportunities or counter-play for more exotic moves and hard to pull off strategies to pay off.
The balance of the game was barely ever touched the game is already pretty fairly balanced yes but there is room to improve it without de-naturing the game or changing any mechanics.
Think about it, every bit of the tech tree used that means instead of big patches or lusted ogies in every games we would have some lusted ogres yes but also room for some sappers, some runes, some humans, heal / exorcism / church even skeletons if we manage to give them a little bit of utility back.
This would be even more entertaining than it already is and amazing to commentate and watch and advertise.
I don't want to change any mechanics or way spells were designed to work because nostalgia is such a big part of that game, most people remember what heal could be used on and what it did they don't remember how bad it was though or how much it healed or cost, tweaking some numbers a bit to try and encourage more of the tree to be used could really be a nice thing.
People love to see these spells used, think about how excited we all are when we see skeletons,or flameshields or heals these spells are so cool and they bring back memories too.
Tricky moves being executed are exciting, we could have a little bit more of these without making lust useless or changing any unit mechanics or spells one bit.
Tweaking mana costs and improve the comfort of use of some spells there and there (heal working like ex for example) these kind of things.
We are talking changes that the average warcraft 2 viewer wont notice at first glance unless he pays close attention to the numbers.