@bijuice I recall when achievements were still kinda new and weird a little game called viva pinata. While it was a creative sandbox of sorts, the achievements essentially guided you to get 'everything' the game had to offer, and it ended up being one of the only 'monster catching' games I actually finished completely and loved.
I think that's probably the way to do it. Freedom to do what you want, but a guide to encourage you to do most of it.