Smart To-do

Add the option in the To-do list to automatically add achievements in the same game, after one is completed.
Similar to how smart downloads work in the most streaming apps.
It could be a pro only feature, considering it will devalue a lot the limit on how many achievements can be active at once in the list, as you would technically need just one per game instead of maybe the whole list.
Maybe even let it automatically recommend you a new game as an optional feature, if it can't add any new achievements, either because you 100%ed the game or because you already had the whole list added.

You can already add all remaining achievements for a game to your to-do list, if you want. (Game page, "..." button.)

Or if you just want to see all of your locked achievements, go to the "achievements" tab on your profile and set the filter to "achievements not won."

Adding everything to your to-do list would just duplicate this.

I know that's a feature already, but I was thinking about using the to-do list more as a "backlog sorted" to just filter out one game for me, between the ones I've started already.
Considering there's games like Payday 2 and Killing Floor 2... Using that feature would fill my list in literally a few seconds (even with the thousand spots). My idea was to be able to have just the next easiest achievement in the to-do list, so then every game would have the same chance to get drawn out, and whenever that achievement is completed, it would just put the next one available on the list.
Sure this can be done manually, as I'm doing it right now (or at least I was until I started to focus just on a few games at once), but it's just tedious for a feature that could be implemented fairly easily