I rather liked this post by Sebastien Lambottin, designer of the naval gameplay for Assassin’s Creed 4, on building up a prototyping toolbox.
For myself, while I haven’t really built up a toolbox of my own prototyping code (something I intend to fix asap, starting with code I’ll cannibalize from System Crash), I have been building up a library of useful Unity assets. Things like minimaps, networking libraries and planetary body generators. They have a different asset on sale every week, and every so often a big sale, just like Steam. And I’m always looking out for stuff to add to my toolbox.
And when I finish SC, one of the rewards I have in mind for myself (after I spend some time vegging out and remembering what it’s like to have weekends free) is permission to spend a good few weeks simply messing about building little prototypes, game jamming. I haven’t done enough of that, either. Time to rectify.