Obviously there are many possible variants with different board sizes and shapes. What should the density of X's O's be, and what constraints on their placement?
I'd say that there should be an equal number of X's and O's, and maybe about one-third neutral spaces. This distribution seems to allow for a decent tactical range. I also believe that the X's and O's should have identical placements relative to each other (some rotation or reflection of the board is the same as the original board with X's and O's inverted). I wouldn't discourage players from experimenting, though; a board with no X's and no O's would be a purely combinatorial game where the last player to move wins, and while the short-term tactical value of a move would be harder to calculate, if at all existent, some players might enjoy that.