Here's a thought: What if when you buy Mikael's Crucible, it starts on CD?
Back when Promote was available as a Summoner Spell, you couldn't use it for the first 5 minutes of a match due to 5-Promote cheese strats.
Presently, Mikael's Crucible makes it really hard for Ashe or Varus to be usable, as their ults will get purged easily. To give them some leeway, I think that when a player finishes constructing a Mikael's Crucible, it should start off on cooldown.
This is just my opinion, though, so take it with a grain of salt.