I feel there should be a preparation which guarantees a specific altar will be present on your dungeon. Given that which god you worship is so critical to your overall strategy, particularly in the harder dungeons, at the moment if you don't get the altar you want, you just have to keep re-rolling the same dungeon again and again, which is boring.
If not this, then there should at least be a preparation to banish a particular altar from your dungeon, giving you a better chance of getting the one you want.
I feel that these would be nice additions to the church preparation list, which currently only has one good prep, (scout: altar) and a bunch of fairly mediocre ones that are rarely used.