As Shonai_Dweller indicated, ArtemiusTheHuman's suggestions are mainly suggestions, and belong in the suggestions forum, where Toady will have a greater chance of acting on them (questions in this thread are read, answered, and forgotten [i.e. no particular action is taken after that], while suggestions are looked at from time to time).
1. Gremlins are the only creatures that can be "recruited" from the wild, and there are quite a few bugs in that implementation. Gorlaks, Plump Helmet Men, and animal people are reasonable extensions of that "recruiting strategy", but will probably wait for a future implementation of animal people tribes and tribal interaction before anything is done with those (together with the bugs affecting the recruiting). Slow learners (trolls, ogres, troglodytes) are more problematic as it would require figuring out how to create a society where they fit in, as dwarven ethics doesn't allow for slavery, and that presumably includes veiled versions of it, such as indenture, caste stratified societies, etc.
2. Founding new religions would be quite rare, or the world would get overrun by new religions popping up, flaring, and fizzling out (and the rate of that might well become a slider control), and that, in extension, means it happening in a player fortress would be very rare. As far as I understand it new religions are intended to be possible to appear during game play, so the question of whether it will be possible in a player fortress basically comes down to how much extra work it would be required to make it possible (cool, but it has to be worth the effort required).
3. Currently not, but it certainly would be reasonable for holy scriptures to act in a way similar to value books. Note, though, that DF is a poly pantheon world generally (like the ancient times in the real world), and so doesn't mesh well with the modern worlds monopolistic monotheistic religions, which are the ones producing the most known holy scriptures (not saying polytheistic religions don't have them too). A scripture trying to gain your support for Artemis would not really do anything about your opinion about Jupiter, while one rooting for Allah would at the same time require you to distance yourself from all other gods (not that it has stopped practitioners from trying to cull favors from multiple monopolistic gods concurrently in the past and present...). However, you can't just have books pulling you towards gods, or you might end up with dorfs who don't have enough hours in the day to pray to them all (which already happens...), so there has to be some balancing mechanism.
4. Currently not, as residents and citizens currently are under player control (residents sometimes under too little control, though), with the recent option of sending them to other settlements (or banish them) being the only exit paths (apart from the old one of more or less accidental death).