You already can create your own libraries with blitz, even blitzbasic 2.x.
Also why not just use gadtools for gui creation? Its super easy and simple under blitz. Alternatively you can use mui or classact/reaction. End results are less bloated too.
And yes, you can use sprites and bobs under amiga mode too.
Remember blitz was designed as a nice, easy to use, efficient language for the amiga and its os.
Half of what youre suggesting wouldnt fit in with that ideal and would be to its detriment.
Also, dont confuse a lack of knowledge with a lack of power. Blitz is indeed quite powerful.