That's a good idea, I think it would be nice to fight the bosses you downed again, but I don't think that fixes the problem I see with leveling. There seem to be a ton of quests to do at low levels but as you get higher and higher, the questing hubs get smaller and smaller.
I played by hitting all the dungeons, which I did because I wanted to get the permanent stat boosts for completing them. I imagine most people will play this way as well. By the time I hit 40ish, I noticed that I literally had a dozen greyed out overland quests compared to a handful of xp quests.
My suggestion is that, why not turn some of the overland quests into high level quests so that you dont have to keep running let's say Permafrost 4 so you can level up to get high enough to take on the boss. Or maybe it's just me and you guys are steamrolling those bosses, but as an SK it's kinda hard. Vox I did at 53, which took me 2 tries. I died the first time because when he casted heal he had just hit me with his fire spell and I never saw it, so he ended up healing to full. The second time I could stop his heal, at least his first one...it was still close, but a lot of fun.
Phinigel at 52 just walked all over me so I will have to try again. Hopefully one level can make the difference, if not it's back to playing the same quests over till I hit 54. Still it's a lot of fun, just saying why not make use of those quests that get greyed out at low levels.
