Yes we could change invisibility cost/effectiveness though even if invisibility is strong I think most of the other spells are in favor of dks.
Haste > Slow (buffing is easier than de-buffing)
Coil > Fireball
Unholy armor > FlameShield (Dk bombs are pretty devastating)
The only spell that dk have that sucks is Skeletons and yeah it could use a slight buff and I mentioned but I wouldn't want to change any of these first
Invis is strong and can change the outcome of a game it is true, but it is not like we never see hasted dks able to do a similar job or even better in some occasions.
I am not saying nerfing haste or invisibility is out of the question though, the way I see it, it is a minor balance issue compared to lust / heal.
I think even with a double mana cost increase on lust, ogre mages will still be way better than paladins lust still will be > to heal because of the ease of use, runes also will be > to exorcism which is fine I think because it wont be that much better, humans do not have to play exactly the same and have the same strength/weaknesses than orc for the game to be balanced.
I would advocate we just try lust / heal changes and see where we have to go from there as I said the more subtle the changes the better we are off when is comes to these things really.
Small steps to make the game even closer to perfection than it is without denaturing it.
Right now as it is we barely ever see a church built for humans if you remove the altar of storm and the church from the available buildings which is very much doable in a custom map it is not like the unbalances are crazy between the two races it is in fact very minimal and I would even argue some player will still prefer running the orcs in that situation.
All the other changes I suggested that are not heal/lust are examples of things we could do from there in the future, but if we are to balance things one day I would advocate doing that using very small steps to try and change the minimum we can at every step of the way