Well I know there is already a thread discussing the sub. features but I'd like to expand on it some more..
Pretty much the only incentive you have have right now to sub is for xp bonus; but once you max out all the stats you want, theres absoultely no reason to sub.
One idea I had is to have the skill island (farming event) available 24/7 for current subs, and they have [for exmaple] 3 hours a week that they can spend on the island. Since everyone is on a different time zone/schedule, they could utilize their 3 hours at their convenience instead of waiting until a mod randomly hosts it. And after they spent their 3 hours on the island, they have to wait until next week to get their 3 hours back. This is just one example, but more features for subs is definitely needed....

RINGS!!! There is already a slot available for them on the equiptment tab... Would be totally awesome to have that completely useless crafting item finally be worth something! We craft these rings for XP and then just dump them in the garbage.. I'm not sure exactly how RS2 utilizes rings but it would be an awesome feature.