I think a lifetime "Achievement" system could be neat

Essentially just a permanent event, where it starts easy like "Earn 1000 coins a minute" and eventually level 30 of that achievement is "Earn 10T coins per minute". Could give benefits for completing, either gems or stones (but a small amount, like 1 stone per level). It's kind of like the milestones for each tier, but it wouldn't be locked to a single tier.