Regarding Inquisition, I have never ever picked it in the current iteration. The faith cost reduction of inquisitors is simply too small to be useful at any stage of the game and the cost of having to wait that your cities lose your religion before gaining the gold bonus from inquisition is too high compared to the (decreasing) gold you get. Plus 2 happiness from spies is very small, especially since you more often than not get your first spy when you or someone else reaches Renaissance era.
All in all, even on its own it's a bad options, made much worse by how good some of the other Enhancers are!
But I do see a very useful "niche" for it -> make it for civs that want their religion "to be left alone", kind of like Spain and the Celts, especially useful if you're surrounded by strong faith civs that will exert plenty of pressure on you either passively or by spreading, but you still want to keep your cities as full of your followers as possible (to keep the benefits from the follower beliefs as high as possible, to avoid religious unrest, to lower the benefits of the AIs religions they gain from every follower in foreign cities). I often play with Spain, Siam and some other civs that quickly get religion/enhance and are very aggressive with religious active&passive spread, and when I found a religion or enhance, I struggle to fight the pressure/spread because those AIs have been at it for a while, they have bigger cities at least in the early-mid game, they've already picked Churches, Orthodoxy, Pagodas, ..., so I'm having to often use inquisitors in my cities to remove foreign religions, but they get very expensive and I'd rather be using that faith for other purposes.
Some brainstorming ideas, some very strong, others less, all intended to be AI friendlynot exploitable by the human player:
1.) Picking it means you suffer no religious pressure and no one can spread their religion to you.
2.) You suffer no religious pressure, but others can use missionaries and GPs to spread their religion to you.
3.) You suffer no religious pressure and others can only use GPs to spread their religion to you (but not missionaries).
4.) Foreign religious pressure is decreased by __% and no one (or perhaps only GPs) can spread their religion to you.
(Maybe some options could be made a bit less powerful by eliminating (like the Celts) or perhaps decreasing the religious pressure you exert.)
5.) Inquisitors are 80% cheaper (75%? Perhaps 50%, but increasingly cheaper with each era?), plus some other small bonuses just so it isn't only about the cheap inquisitors.