Hey, i really like the effort you have put into this map so far, seems to be good for pvp, though it still needs to be worked on more. We would like you to find a name/theme for the map so it's easier to give you feedback.
I will anyways point out some of the features you can work in and improve:
The shapes of the starter islands don't seem natural, maybe making them a bit bigger with irregular shapes would work.
Also, it would look better if you replaced the vanilla trees you used on those islands to custom trees.
The painting of the bottom part of the islands are not elaborated enough. I would suggest to find a palete of different blocks and paint the islands according to it.
About the structures in the mid, i can't really tell what it's supposed to be as you didn't clarify the theme of your map, but something i can say is that it lacks a bit of details, i would add depth on some walls and use more blocks variation, also, would like you to improve the interior.
I would also remove the ladder structure as i don't think a player would climb a ladder tower just to loot a single chest and it would allow some people to skybase.
More about the gameplay would be that there are some islands further from the mid or another islands than others, you need to measure the distance between islands and moving them making sure it doesn't give more advantage to some players.
If you really want your map to be accepted please keep working on it according to the feedback we have been giving you. This is a good start! :)