A few other people really hit the nail on the head.

The problem with combining both is that they're both limited by each other. For example, in high-end PVE, it would be really cool to have a lot of horrendously powerful abilities that make you feel all-powerful (for example), but that won't fit in a PVP environment well. If you focus too much on PVP, lots of elements and abilities are added that just don't serve much purpose in PVE.

If I were you, I'd say either PVE only, or spend a few months trying to come up with a good system to integrate both before starting. Planning is good!