I won't add new cave until FoxBoss has been killed a few times.
I see no reason to add new content when the current content hasn't been beaten.
I will start off by saying that this server is great and the only private server I have ever donated to since you have given so much to the mir community as a whole it C+ and this.
However this statement only applies to those that group up and go for bosses, which is the same select few that always do such on every mir server.
The majority of mir are the scrubs that just hunt in caves and group up the odd time in harder caves, casually playing the game (I would be surprised if they have even killed every boss on a low rate server before). I don't play that often and most likely wont kill any boss mobs for some time so in terms of playing the game PB/PT are really all there is on the server which is better solo than group.
If I was to compare it to Gamepot there would be PT/PB/Hell Cave/Ancient Caves/Ancient Ruins/Vipers/Red Cave/Lunar for my level. I know these files are still being built up but the issue right now is they have less content than what the majority of the player base had 5-6 years ago on an official server.
A good target would be to get it to this amount of content in a reasonable amount of time then build up from there.