I decided to try out your server for a bit, see how things went, and I have a number of thungs to comment that may help you. 

1.  50,000 slots, really?  Only the most popular servers have that many slots and more, if you have a pad sever then I would downgrade the slot list from 50,000 to maybe 20, and if the playercount exceeds 20 then upgrade it, dont expect that many players to get on your server, espicially scince its not known very much, trying to help you save money on that.
2.  A way to expand an island substantially is to use slabs, which I used, but every time I walk on the slabs it (the server) kicks me for flying, a way to overcome this, is to remove the anti-fly plugin and use other plugins that dont allow other players to build outside their areas without permission first, I can name 10 off my head that would do this, but you as a server owner need to work on this, and Ill leave that up to you.  If players keep getting kicked for flying, then what's the point of rejoining the server.

3.  What if players dont want to play skyblock?  Dont just limit youself to just one idea, keep moving with others.  Take a look at all the best servers, they have minigames, appeasing looks, sometimes appetizing, but they attract thousands of players at any given time.  Include other types of skyblocks, here is one that is interesting, and here is another that can work out well.

4.  I was looking around your spawn and it looks nice, but something I saw on there, which ive seen on far too many servers that have failed, are two things:  Apply for Mod, work for Admin, and hosting drop parties.  If your intention for this server is skyblock, not factions or regular survival, then why host a drop party?  Also many troublemakers will want to apply for mod so that they could hop to many places and wreck havok, do not let them apply, this always leds to arguments, and ruins the game for a lot of people.  Only give out special ranks to the ones that follow the rules, that are nice, helpful, and want to help others, I know that this will work, this ensures that the most deserving get the ranks, and not some immature kid who would wreck everything given the chance.

5.  Lastly, and this is purely my opinion, you use a bit too much quartz in your spawn,  I know what you are going for, dont get me wrong, but the quartz overload is a bit much.  Maybe make a schemtic of the spawn and put it in an air world, not superflat, this shows laziness, and makes it look unprofessional.  Make players invincible or have the ability to not take any damage in spawn, so falls are not lethat, (which I have tested) and maybe add some parkour. 

Look for interests that everyone has and try to incorportae them into your server, it should boost popularty and help you.  And I really do reccommend to lower that slot list, your server isnt that popular yet, but im sure it will get there in time.