Handling Hi voltage and very low current (10 mA) will need to prevent arc and corona effects on my PCB?
I'd also take a look at the IPC 2221[edit: Electrical Clearances] standards before going any further with the PCB design. For a high voltage PCB, the one thing that matters the most is the material selected for the core/prepreg. Depending on the availability and budget, you can go with extra overcoats and better alternatives/versions of FR4. Also, consider a Kapton Overcoat if possible.