I think that the top area could stay flat and the left over polys (once the character is modelled) could be used on other details that you may be able to add, like a few stones on the surface of the ground, some single plane pieces of grass and so on.
One thing that I believe is a MUST, is to not make the top section completely round, it should be rough looking around the edges like the rock has been ripped from the earth, I guess it would just give more of a realistic (in game terms) feel.
I decided I'd do a quick MSPAINT draw up of what I meant with the edges...pictures are far better than words after all :P
(I am obviously a pro at paint!
)
This kindof reminds me of something from zelda windwaker and Mario 64, so in other words, I LIKE! :P