Personally, the servers which i played were mostly successful because they were simple. Servers like foxlv or Lost paradise online, didn't have a lot of mazes, which i think is a good thing. Servers who add a shit ton of mazes like the main mazes (fc/ds/dw/ca/bd) and like +7 more unique mazes, i think thats too much, since players get confused and simply ignore most of the unique mazes.
The other thing i liked about ca in foxlv, that after CA ended, on top of the bar, on system. It showed which player killed the most players, which player died the most times etc.
That thing really encourages players to pk, where the stats actually matter. Same thing in LPO, there was a ranking for most kills in certain mazes, and those top players would get rewards.
So i think keeping a simple but medium/easy server is best, then after people get the hang of it, add more content, more lvl's, better EQ, better gems, pirata cup or smth and so on.
And 1 more thing, personally, i don't like servers where you run sonic super fast, and you only have to have a good ping,fps and only fb's to catch them, i think thats retarded. Keeping it simple movespeed is normal, that is normal pk which i think people enjoy.