Use ideas:
Put a gravel or concrete floor in depending your budget. Or leave it dirt, our barns have dirt floors, it hasn't hurt us so far.
On either side you could stick a row of stalls in using the modular stall units, that way you have your stalls you need.
I would use the lean to side for hay, horse trailers, implements, etc, etc. You could easily run an electrical circuit out there to put up some better lighting and have enough power available to close off one section and make a nice workshop. Consider running phone as well when you do it, having a phone in the shop/barn can be very handy.