After removing a wall how to repair an odd shaped section of hardwood floor
Going side-to-side with a contrasting prefinished flooring can work.  When I did this at my house, the flooring runs were perpendicular to the closet, and of course, they didn't lineup.  I made a diagonal section 4ft wide section, across the whole room.

They make floor medallions that could disguise the transition.  You would have to extend new flooring to either wall.   

Ceramic tile would be another option.