The problem is, I don't see enough staple religions to base other religions off of. Harvest, Music/Poetry, Science, Sun/Moon/Astral Bodies, and Seasons, etc. Each one is a loose religion that the more defined ones can come from. Nature may be a catch-all for Sun and Moon worship, but how many people actually branch out and do that? Most are just tree-hugger Watcher-wannabes.
A lot of RP comes from motivation, imo, and that comes from Race and religion. Mystic hardly covers the more conventional aspects of science or just knowledge as a whole (such would be good for a Herald IMM, if it was put in). There is no outlet for Harvest or Seasonal religions, because those don't fit anything but Church, which has become a pseudo-Christian "Church" without the loose definition it used to have pre-Crusaders. That might as well be its own distinct religion now.
This suggesstion does not come without having seen the possibility of NPC-based religions. It works. It works well! IMMs still run the show, but they are more like Demi-Gods (which makes the transitions between various IMMs seem more easily believed and requires less explanation). As Demi-Gods or Avatars of the Gods, they are the Voice/Word of said deity/religion. This adds a ton of dimensions and would even allow multiple Gods in a single religion (that would take more coding, but would allow one Sun worshipper to be about rising sun, or life, and another about the setting sun, or death, etc).
Overall, I just see it as a great way to expand on religion - which doesn't play nearly as much of a role as I feel, imo, it should. Of course, Race plays little difference either, which I would love to see fixed as well via a Crusade-like system for "Noble" members of a Race to declare, giving a bonus in their realm to their own, etc. Of course, that is another idea altogether and another set of code.
Anyhow, just my thoughts on the matter.
oh...and as for the subject that this whole thread was started for.
I really don't feel that we need more religions or deities, we already have quite a few as there are. If you want to pick the religion that is closest to what you want, and rp from there. I don't think ever had a character that RPed a by-the-book religion, just because I generally find them too constraining.
While I agree that the RP possibilities of having more class/race specific deities would be exciting and interesting, I feel that there is enough freedom in the current setup. There's plenty to do with RP as it is. Although not everyone may participate in the RP (though I see plenty of it), the chances are out there.
On a less RP based angle, I think Behrens has enough to code as it is. Although you say it wouldn't affect the current balance there would need to be bless (and bless weapon) affects that would have to be tested (yes I know these aren't terribly major, but the affects are there).