Wow...those are some questions w/ some LONG ass answers.
1. Well Faiz was so an open-ended ending, that no one is really quite sure what happenes after the final episode. Does everyone live in peace? Does Taku keep fighting as Faiz? Does he EVER finally clip his nails? Who the hell knows. I'd like to think he's STILL on that hill moping and trying to figure out what he's going to do.
As for Agito, it's been awhile since I've seen the ending, so that's a question better left to others who remember it better than I.
2) Each rouse card has a different "power" based on the Undead that is sealed inside it.
This site has a pretty good list of the cards and what they do. Although it is FAAAAAR from complete. I believe it was someone's site from JH and they stopped updating around the middle of the series or so. But it gives you a good idea of a starting point, since the name of the cards are pretty explanitory of what the "power" is.
3) I don't think ANY KR series can be summed up in a 'brief' review.
Eqipment/Weapons for Ryuki,Faiz,Blade....they've ALL been covered in various websites & messageboards. Finding the information by doing a little bit of research shouldn't be too hard. Again, this would be a SUPER long list for someone to just list and make.
Good luck in your fanfic.
-RF