I read the entry in The Inner Planes for the Positive Energy Plane last night following up a fit of inspiration for a character ... it sucks. There's absolutely nothing there. Let's renovate.
To sum up the canon: It's an infinite plane of nothing but pure energy - this energy is what gives everything in the multiverse life, creativity, existence, etc. It's so much energy that anything that goes there without positive energy protection is disintegrated. As such, there's nothing there. One site is listed - the obligatory fortress created by a mage which manages to survive using lots of magic.
For inhabitants, there are xag-ya, who are ineffable, and ravids, which brings any lifeless object it comes into contact with life but aren't very intelligent and don't form societies. There aren't any permanent Powers, civilisations, or basically anything - although some powers and high level mages visit to create life.
One interesting property is that there's instantaneous teleportation throughout the plane just by thinking of the location. If there was anywhere to visit, this would make for interesting dynamics.
Also, the energy is blinding, but sense of touch is enhanced. There's no air, but the positive energy heals you even if you start dying of suffocation, starvation, etc - although of course that doesn't work if you have positive energy protection. Wounds are healed instantly, but you die from too much energy soon afterwards anyway.
---
For this renovation there's going to be some challenges - a good part of the concept of this plane is the irony that it's the plane of life and yet it's impossible to live there. I like that, but I don't know how to reconcile that with making it an interesting place to visit. Hopefully someone else has a few ideas.
Well, the only thing that does occur to me is that non-corporeal beings of positive energy should be able to live here fine, just like non-corporeal beings of negative energy (wraiths, ghosts) thrive in the negative energy plane. Are there any other canonical non-corporal creatures like this other than the (rather bland) ones already listed?
The only reason that the Negative Energy plane is more interesting than the positive is that it has undead, and most undead can't survive there without protection in any case (by my reading it should only be non-corporeal ones that can). Is it just that the undead are more tenacious than the living? There are supposedly plenty of fortresses and such created by liches and powerful vampires there - why don't the living show the same sort of interest in the source of their life?
And just as a curiosity - what happens when you take something powered by negative energy and drop them in the Positive Energy plane? The reverse of what happens in the negative with living creatures - so instead of wasting away to undeath, the undead comes alive again? If unlife starts to drag you down, perhaps a trip to Positive Energy is in order - although I guess much like the negative energy plane the effects would be unpredictable.
... but yes, overall I'm not sure of a good direction for this - looking forward to seeing other ideas.
I could easily see a location filled with people who have figured out a way to avoid death and, as a result, losing their soul to whichever afterlife they're destined. So, whatever this place is, it's filled with folks who have a really good incentive to avoid whatever awaits them in the afterlife. People who have made bargains with fiends, people trying to avoid paying for a life of evil. That sort of thing. A good destination for a party to go to apprehend somebody. I'll think about this some more at work today.