You can check out Gallia, it's some fresh and new RP opportunity and is actually not as emtpy as so many people think.
Then there is the neighbor and ally of Liberty, Bretonia, it has a lot of active factions and uprising ones. It's rather full, you have an enemy and pirate groups to shoot at.
Rheinland is somewhat a mystery to me. It's normally just Hamburg NB and Omega 7, where stuff is going on. Though you will be in the same KRIEEG thing as on the Liberty side. The RP idea is easy to get and there are many non-lolwut indys.
Kusari is almost empty most of the time, but be prepared for RPRPRP, when you meet someone ( not a powertrader ). There are some lols, but also many people ready to RP with you, by both talking and with guns.
Corsairs only, when joining off. faction; same for the Outcasts.
Zoners have an open environment of RP, many people like you or don't shoot you. They have 3 faction worth of joining.
Did I forget something ? Yes, if you want to try out some 'exotic' RP, get a nomad trial char and begin RP a nommie. If you are good, you get a chance to join the Keepers and have fun shooting stuff, lots of stuff.