So my idea is player must reach specified skyblock lvl, then when a player reach it they can open a portal to an island, like real one. But player must ofc create it by they're self, maybe using glowstone and water. When player reach the specified lvl they only can create the portal and not suddenly a portal appear from knowhere. 1.13 Introduce us to fish mob and others aquatic stuff, so near the island is surrounded by beutiful coral, and maybe others like those sinking ship and small temple.
I like the idea of having a portal to another world. The issues I find with this is the fact having another world would likely impact performance in some way. The server is 1.12.2 so you won't be able to have 1.13 without going to a separate server anyway. It may take some effort to integrate the levelling system to unlock a certain permission to create a portal as well. If there is some way to have another world that allows players to chat with skyblock people yet have a different game mode experience and it doesn't impact performance then I think it would be pretty fun.
I do like the idea, but I regret to inform you that this is a skyblock server, not a waterblock server. HOWEVER, do not lose hope. I think this isn't too bad an idea, but may not pair well with how the servers currently are. Though due to the near limitless creative-aspects of the game, you could even go so far as to transform your own island into what you're describing. I don't not like it, I just don't know where it would fit into the server. Neutral.
Neutral we run 1.12.2 like Krissy said we would have to update our server and possible introduce some lag along with it