Probably come down to how to keep it balanced. From a realistic point of view I'd see this as very doable, since a settler's whole goal is to found a new city, although at the same time I could see an argument for 'patriotism' or some such as being a reason they can't be converted. Enemy AI for them would have to be significantly improved, or else it'd be far too easy to simply snipe settlers from them. Not sure which side of the fence I'd fall on with captured settlers. At the same time though, I could easily see a side benefit of a Wonder or Civ UA being the ability to convert religious units (or even apply this to other full strength great people). The amount of production of a great engineer on normal speed is calculated using the formula.

Like was said, I wouldn't really trust a captured religious unit. A great tile from a captured unit would almost have to work at full strength or it'd be fairly underpowered, but possibly only give half benefits while you're still at war with the civ you captured it from? Not quite sure about this one and how to balance it while keeping it 'simple'. A captured Great Scientist, Engineer, etc would only be able to use their 'special' ability at half strength From a realistic point of view, I couldn't see Great Generals or Great Admirals being capturable, but it'd be pretty neat for other ones to be.

Great general civ 5