In my 7 years of RO playing (hehe... Get it.... Role playing? K nvm) i have concluded the following in my opinion:
WoE/ BG players/ PvP players: 75%
Actual leveling/ PvM: 10%
Players who play custom servers that take an immense amount of work to develop yet die in 2 months: 15%
(Did that add up to 100%? Smh)
If you don't have woe you won't have a population. Guild packs will attract them, yet they will leave your server after another woe server opens with a g pack and your server will die THUS the short cycle of a private RO sever. But this is highly debatable I'm sure, this is just what I've been seeing.
tl;dr don't go too crazy with the customize button. Add a nice BG system + woe treasure and your server will do fine.