I'm surprised this plugin doesn't already exist given the amount of reward type plugins around, but it doesn't. It's nice and simple too. IMO.

Main

What this plugin would achieve is to reward players for their time spent on the servers by gradually adding them to usergroups which give them more permissions. Or just giving them more permissions directly, but I think the usergroup method might be better as you get more benefit by doing it this way (multiple permissions applied at once).

The case example below is Backpacks, but this could apply to many other plugins that use permissions (or custom permissions) to grant access to higher levels. Other examples are NTeleportation,  Extended Recycler, Rust Kits, Sync Pipes, and so on.

The data for each player should be held in a data json so that at each wipe it can be cleared down and the players all start on a level playing field again. This incentivises them to keep playing to achieve their previous levels and convenience.

Ideally, there would be a text message to alert the player that they've achieved a new level, plus some additional info:-

"LEVEL UP. You now have access to:-"
"More backpack slots! (12)"
"More homes! (8)"
"Your next level up is after 8 more hours of play time."

Example:

With Backpacks, the initial permission level would be backpacks.use.1 which gives players access to a single row of slots in the backpack. As they spend more hours on the server, the proposed plugin would then give them a new permission backpacks.use.2 which gives them two rows of slots in backpacks. And so on.

Conclusion

I hope that someone will pick this plugin idea up. As I said above I'm surprised it doesn't already exist as it adds a new level of incentivisation for players to stay on your server. If these perks are already available on a server for vip donators, this is the fairest way of giving the same level of perks to players who have time but not money. There are a lot of them about.