Removing output voltage ripple in high voltage outputs
You should consider higher frequency switching operations and a means to minimize filtering needs.  Look into interleaving options (running n converters shifted by 360/n electrical degrees for their PWM periods) as well - this will have a similar effect as a single converter running at n*f frequency and can provide a modular/scalable solution.  Rob B.'s suggestion was for a 2x interleaved system, but it's extendable to many stages/systems.

Depending on the application... an important point to consider for high-voltage supplies is their performance under arcing conditions.  You may particularly want to minimize filter energy for this purpose as much as size/cost/weight reasons.