Correct me if I'm mistaken, but I believe it was the same for the old system. If you combat log, you lose your XP. Also, I don't really see how making a room Is going to work out. If a few people needed to get XP at the same time, they'll both be stuck in the room, and well... It'll take us back to the problem we began with - People stealing XP. And if we were to make an individual room for the each person, well... Kitpvp has grown quite a bit. The amount of people on nowadays are incredible. So obviously the rooms wouldn't work inside the map, and instead will end up being taken out of the map, where that one person will find a way to get out of the room and into the real world, where more problems will arise, blah blah blah. But maybe I'm not getting a proper understanding of your idea. Could you possibly elaborate?
I personally like the new system because I feel as though it makes it easier to get XP. Some may argue, that you can save up bottles but you can also save up this magical thing known as money, and buy XP when you need to. That's just my opinion...
Also, I don't think the lava room is coming back. I think there was something wrong with it that allowed players to break blocks. Not sure though.