I managed to fit DRV8818 circuitry, a PowerPeg heatsink, TVS protection, and DIP headers onto this tiny PCB. This driver can operate steady at 3.4 amps peak without overheating! With a larger driver new printer styles are possible using NEMA 23 and 34 motors.
Why do you limit yourself on a single "board design"? With the recent popularity of raiser "solution" the bottom surface is also avaibale for either heatsink mounting or electrolitic capacitor mount. So why don't you explore a low footprint but "multi level stack" type driver?