I think all bonuses when idle should be removed and replaced with something else. Its annoying, and I never want such buildings. It feels half-baked to me.
I agree that a production warning (with an option to toggle it on/off in options) would be nice. Warlock: Master of the Arcane does this so well, that sometimes I feel the game is playing itself
I highly dislike that mechanic as well. Even more than the Magic Leveling system. I never ever ever build anything that only adds when the queue is idle, and would only consider doing so very late game when there is nothing else for that city to build again, ever.
Have those buildings give us a "construction" option for Wealth, Research, and Mana, and have any building that used to give a "bonus when idle" improve the production to resource ratio thereof. I would rather explicitly apply production to the generation of a resource than implicitly gain it by not spending my production on something else.