Has anyone ever created a server where you play as a mob? It could prove quite interesting to be a mob with their stats and skills (and obviously ability to use SS/BSS and buy/sell stuff etc).

I imagine at the start you could pick one of the mob races (humanoids, plants, insects, animals, beasts, spirits etc.) and then you would start as level 1 mob from that category. If there are more than one, you can choose which one specifically you want. You could for example choose to be an Orc, so you start as a lvl 1 "Orc" (literally) and as you reach certain level, you would transform into "kaboo orc" then as more variants of orcs are available during leveling, you could do kind of occupation change and choose between "kaboo orc fighter" and "kaboo orc archer" etc. In the end you would become one of the Ketra Orc variants.

All mobs would get stat boost every level above their default one (for example lvl 40 breka orc will be better than lvl 35 breka orc) to compensate for the fact that some mobs have fewer variants than others. If there is a race that doesnt have lvl 1 mob to start with, player will get the appearance of the mob with the lowest level of the race, skills will be disabled, stats will be heavily nerfed to be as strong as other lvl 1 characters. As the player levels up, the mob will get stronger and stronger and at the default level the mob will get its skills and stats. With further leveling the stats will improve like any other mob.

This kind of server would probably require GM buffer as mobs have very few buffs and playing without them would be a pain and also a GM shop as mobs cant craft. Manor could still be done normally.

dude even ncsoft could not fix some small bugs since c4 untill h5, i doubt random developer could do this challenging stuff

As sad as it is, often indie developers can make much better games than huge AAA studios. And even more often individuals can turn a crappy game into something completely different and awesome. Besides, NCSOFT only recently managed to ban bots while private servers were mostly bot free for ages.

Damn dude you have the darndest ideas That being said, I'd totally try it

Also if I could take it one step further, maybe this server could have a 'Tower Defense' feature - basically one part of the server would play as regular characters and the other one would control hoards of mobs, and every now and then there'd be a 'siege' where the 'mobs' would go in waves at some place and their goal would be to reach the end for something. The characters would be trying to stop them obviously. But I guess I'm letting my imagination go too far at this point

Nonsense. There is always room for event like this