To be able to pump out a unit with max armor, max weapon, all accessories and mounts and the most expensive traits, for a flat fee of 40 mana once a day, is very OP.  I think it would be much better if you had to pay 20% of the remaining production cost in mana (rounded up), so you could still get out those mega expensive units, but it just would be a lot further away from "virtually free".

What do people think of something like this?  With the Rush ability, which allows new production for gold, and Call to Arms, which allows new units from mana, for units you would have a choice of using gold or mana, but I think both should be somewhat related to the production value remaining.  Also, it may be worth considering Call to Arms having a longer cooldown than 1 season.