This topic came up as part of another post, and I have heard it echoed on Facebook, but I am going to create a new topic for this idea.
1) The game is harder that it was before, causing players to play in worlds that they have already finished because they cannot survive three rounds in their current world.
2) Shards mean that you will be getting new, low-level heroes going your team as you progress and need to go back to easier worlds to level them.
So I think that you need to make replaying the early worlds more fun.
My suggestion is that the worlds reset. Maybe 30 days after you complete a world, the stars for completing it all get removed. That way if you need to go back to that world for some reason, at least you have a goal. Otherwise the grinding of playing the same low-level worlds over and over as you wait to level up gets boring.
I requested the same thing via a support ticket and was told it would be too difficult to reset them. But maybe if enough players request this they could make it happen. I totally agree that it's boring and without having a reward makes it less enjoyable to play.
Unfortunately we can't implement this suggestion due to technical issues. But we're working on some other ways to quickly and easily loot things from the worlds. Sneak peak: Quick Fight! :) You won't have to sit through out the battle if you've already completed it with 3-stars. :) I know it's not the same but I hope it will be a nice feature to have.