I think it would be interesting to consider a second Spiral Knights server for players who paid a monthly subscription fee, which removed energy from the game, replacing it with other limitations as necessary. (Probably purchasing boosters and equipment slots with crowns, and giving characters 3 lives per run - things of that sort.)
This server would run on a completely separate economy, and would allow players to play Spiral Knights as a casual action game. Players would be unable to accelerate their gearing by paying more money, but they'd have the opportunity to play as much as they want.
Personally, I think such a server would be what it would take for me to get to spend money on the game.
Players could probably organize runs across servers, with some amusing restrictions. (Could subscription servers lend free players their lives to resurrect? Probably. But they couldn't tab them. They could probably open energy doors with a payment of crowns based on the energy-server's going CE rate.)
The specifics aren't terribly important, I'm just interested in discussing how people would feel about a subscription server that blows the limits of the game off, but puts you in a different economy. (You can't trade with players on the energy server.) We can probably assume that the price point will be "low enough that you can afford it".
This would be nice.