Iron pulleys make for a lovely smooth transmission on belt drive machines..
..a bit of flywheel effect never goes amiss in some machines if you can find somewhere to include the ‘mass’..
..a great material for making a base to mount a toolpost straight to the cross-slide..
I’d also have a look through Hemingway’s ‘Work Holding and Positioning’ section, lots of stuff there, if it suits the work you do, I guess..
..or what about..
..a tilting table / compound angle base..?
..a couple of big ‘squares’ / cubes / cylinders / angle- & vee blocks..(45 & 60/30?
..heavy vice base/workholding table for a drill-press..