I've been thinking about this for a bit.
Player storage has always been a huge issue for RoF. In fact, extra storage space has probably been one of the most-requested features since the server was launched (maybe after Deathrun and Cold Water). I suggested it two times myself--
with my last suggestion being almost an almost desperate cry for more inventory space.
You yourself helped fixing this problem when you added plots to RoF-- essentially unlimited storage, available at a price of 3000 Cookies for any Expert+. This completely solved the issue of lack of storage for high-ranked players. I do agree with you that player storage has always been a major issue-- Not just on RoF, but in Minecraft in general. Players have way too little storage space and Mojang honestly should've added a vanilla way of increasing it ages ago.
I believe that it would be a good idea to give players a slight increase of enderchest storage space as they go through the ranks (which is possible with
this plugin). They would get one extra row with Apprentice, one more with Journeyman, and one more with Expert (for free). If they'd still need storage after this, they would have to unlock plots.
This way, we solve the storage issue for lower-ranked players without removing the incentive to buy plots. I don't think players should get an insane amount of storage or vaults, but they should get a slight increase in storage.
What do you think about that?
EDIT: Seems like this isn't really compatible with the current way gamemode inventories/enderchests work + most other players/staff members don't actually seem to be in favor of this. I'd still like to implement something like this, but unsure of how to do that.