I am sure I wont be the first person to suggest this however I've been a bit frustrated with the achievement system on steam.

So many games that I love have achievements that are online only, meaning that any game older than a few years may be near to impossible to get all of the unlocks.

I think a category system for achievements would work much better.

For example:
When you click on your unlocked achievements you have 3 tabs: Overall, Offline and Online Achievements.

I understand this might get quite tricky with some games that offer some single player modes with online elements so it may be the case that when a game reaches a state where online achievements are no longer fee-sable to be obtained that games online achievements can be marked as no longer required to 100% the game.

Note: This is my first ever suggestion so if I have posted it in the wrong place please let me know and Ill move it!