Suggestion list

1) It would be very wise to convert your client files to a Jar file for much easier access to the client for players, or a webclient should be created for another way of easy access for players. This will easily bump the up the average player amount.

2) Create a crystal key chest(just like barrows chest but with crystal keys)

3) Add more "rares" for a better economy. I found you can do this by taking a normal item that usually has no stats(like boxing gloves for example) then bumping the stats to make them fairly good, and then adding them to a boss drop to make them rare. This is an easy way to make the economy better. You can do this for other items too like:
-Gadderhammer/Meat Tenderiser
-Anger Weapons
-Scarfs
-Bow Sword
-Etc...

4) Fix the "Teleport to: Current slayer tab" button in the quest tab.

5) Clean up barrows(guthans and torag are all bunched up together)

6) Custom boss? 

7) Make the overall server more realistic. This is a small suggestion, this would be a very big update and would change the entire flow of the server. By more realistic I mean:
-Lower experience rates so that skilling would take much longer, and keep players more occupied for a longer time. This would also make it so that skilling items are much more valuable.
-Remove all tradeable items from shops. This would make it so that all skilling items/combat items are much more valuable. Players won't be able to just go and buy a dragon scimitar from a shop, they will have to buy it off another player. This flows well with the low experience rate because it makes each tier weapon much more relevant to the game, creating more value for the item. This also adds to the rare drop list and helps the economy.
-Lower drop rates to make rares 10x more valuable.

8. Take out barrows whip-lava whip in slayer shop and make them rare drops. Their point price is very high and making them a rare drop is just as easy/will work just as good and adds variety to the server.

My head is always filled with ideas and I will keep writing them down. Glad to help!

--Austin