Now if we were to make any changes:
Overall if we wanted to change the balance without breaking the game of transforming it into something else increasing the mana cost of lust is a good starting point that is true.
Though 150 is a very big change I would start with something around 80 100 and if it is not enough grow up from there 125 etc... It is very well possible that 150 would be the final value.
But the idea would be to make the minimum amount of changes possible you do not want to overshoot it.
Slow however is already a good spell that does not cost too much it is used and has value to it.
I would focus on slowly changing something that is not used as much as it could like heal or exorcism and change the mana cost of that so that it has more value to it.
The end goal would be to have changed so little people coming back to the game feel like the game is the same.
Now honestly the way the game is right now is fine by me I am having fun playing it the way it is.
The problem is that changing the mana costs means changing the game for everyone else.
We could use a second server to try up the changes and all but that is a lot of work to change something that is not that far from being perfect already and that most of us already love.
Maybe if someday blizzard makes a remaster of the game they will make these changes.
That said I am not against trying things if it is done very carefully but I can definitely agree with what IL said.
There are things that would help the game much more like having a match-making system for 1v1 as well as team games, also your clan war thing could be a good addition and what you said about adding the mini-maps in the lobby all these changes seem to have much more added value to them and are less risky to try because these changes are straight improvements for everyone.