Gree: Why not create a new building that generates one gold bar a day or every 48 hours. Program this for to have two requirements to get the building.
1. Must be Level 300 or higher and
2. Make it cost $1,000,000,000
This will do a few things. It will spur interest in accumulating cash, causing people at higher levels to raid or at least use the cash they've saved and make it worthwhile to advance to level 300. Right now, people have no desire to advance in levels because it's a disadvantage in World Domination events to be at a higher level. Perhaps you can even program this to provide automatic upgrades based on level advancement, not cash purchases. For example, at Level 310, the building automatically upgrades to Level 2, producing say two bars of gold every 24 hours, at Level 320, you'd get three gold bars, etc. This would be about the same as what we get, or used to get, from watching video for free gold, and it would be a nice bone to throw to your loyal, long time players.
This would reward long time players and give newer players an exciting goal to shoot for. Suggest you name it Fort Knox!
Come on Gree... what do you have to loose! Just say YES!
