I have an idea that I have been thinking about for a little bit, so I finally decided to make an account for this forum and post it.
My idea is that in order to push players that don't even bother trying to get the achievements, or just make it worth wild for the people that try for achievements anyways, there should be gem rewards that vary in amount depending on the challenge of the achievement. For example, the First Blood Achievement would reward you with 5 gems, while the achievement that requires you to kill 10,000 enemies in endless mode would award you with perhaps 800 or more. It definitely makes sense, especially with items being so helpful in endless mode. There could be other rewards, such as just an item. However the hardest achievements could massively rewards you, perhaps once you complete all the achievements you unlock a multiplier that multiplies the amount of gems you get in levels or even an exclusive hero. I was hoping that for Frontiers that would add that, like completing all of the achievements would award you with a hero (Vez'nan but he's in origins so nvm).
If you guys could tell me what you think about the idea, I might have over looked a huge flaw or something, but I just thought it would be a cool idea.