As this is one of my favorite topics and since I'm feeling like organizing, here are a list of the threads containing planes that have been "renovated". Please let me know if there are any new threads or ones that I have missed.
General information
/forum/inner-planes-resource-index
General associations (for future renovations)
/forum/brainstorming-elements
OUTER PLANES
Acheron:
/forum/planar-renovation-project-acheron
Arborea:
/forum/planar-renovation-project-sort-arborea
Arcadia:
/forum/planar-renovation-project-arcadia
Beastlands:
/forum/planar-renovation-project-beastlands
Bytopia:
/forum/planar-renovation-project-bytopia
Carceri:
/forum/planar-renovation-project-carceri
Gehenna:
/forum/planar-renovation-project-gehenna
Grey Wastes:
/forum/adding-color-grey-wastes
Limbo:
/forum/planar-renovation-project-limbo
/forum/limbo-renovating-residents
Outlands (actually a bit of an overhaul to a Western theme):
/forum/wild-west-planes
Pandemonium:
/forum/adding-color-pandemonium
INNER PLANES
Para-elemental Plane of Ice:
/forum/project-ice-fleshing-out-paraplane-ice
Para-elemental Plane of Ooze:
/forum/planar-renovation-project-paraelemental...
Positive Energy Plane:
/forum/planar-renovation-project-positive-ener...
Quasi-elemental Plane of Lightning:
/forum/planar-renovation-project-quasielementa...
Quasi-elemental Plane of Vacuum:
/forum/planar-renovation-project-vacuum
I would recommend considering the following things when redefining/refining a plane:
REQUIRED
*Unique overall theme(s)
DESIRABLE
*Conflict within the plane - e.g. on Arcadia (after renovation) we had the different groups with their different plans to achieve Utopia in conflict with each other
This doesn't have to be an all out war; just a difference of opinion on the main theme of the plane. This provides internal adventure seeds that aren't on the level of "save the multiverse...again"
*Unique terrain
*At least one unique representative species (preferably one that embodies one or more themes of the plane)
*Reason(s) for PCs to visit
Sure we can always fall back on the standard themes
-Stop the (evil) plot
-Assist the injured or desperate (good) being
-Rescue the (good) individual from the (evil) plane
-Drive the (evil) intruder from the (good) plane
-Resolve conflict between residents of the (good) plane
-Get information from resident
-Get the item (often randomly placed) there
But I also try (often unsuccessfully) to give the PCs a reason to visit (e.g. they want to experience the famous gambling dens of Gehenna, they want to get their future told by the Norns, etc.).
Sometimes this is tough (e.g. aside from being suicidal, I can't think of many reasons to be a tourist in the Abyss - I'm sure there are some reasons, but not many)
USEFUL TOOLS TO DEFINE A PLANE
*Philosophical opposite(s) - I included this because I often get inspiration for a vaguely defined plane when I have a clear picture of what its opposing plane(s) represent