I have a couple of ideas concerning gems. Since you've paid real money for them, I'll be nice if you can make use of them longer.
- when you spend gems for an item (in the market or for upgrade) you should get them back when you sell that item.
- Add an option to restart the game. As your characters comes back to level 1, you should get back all the gems spent in the previous game.
And, please, add an option to buy the second skill points even after training. Some of my characters are not fully upgraded and I (we) don't think to delete the game in order to start for level one again.
In my opinion you'll be more propelled in buying gems, at least I'll be more propelled in buying gems. For example, I could decide to invest in a mega pack, play with a strategy, then restart with another kind of building.