A lot of other free games reward players with upgrades, coins, gems, special items, badges and equipment etc.
I don't know what would work. Just brainstorming.
Play until day 8 and gain a free Fortress upgrade.
Play until day 10 and gain a free factory upgrade.
Play until day 12 and gain 1 free artillery.
Play until day 20 and gain 1 special unit not available for regular production. e.g. Sapper Unit used to destroy fortresses
In the scheme of things, these are small but they are some reward for those of us who stick around and play.
The longer people play, ( don't have any data to back this idea) I guess the greater chance they will purchase GM or click mobile advertising.
These would not affect the spending of GM or via mobile the watching of ads. People will still do that.
Maybe these types of rewards could be limited to or customised for selected maps.
For those who play seriously, the games can be long, so what to do for games that go for months I am not sure.
Maybe need to stretch out the milestones when rewards appear.