I am a little unclear as to why GPS is unsuitable for your needs.
Are you using GPS?
Are you flying inside?
Why do you say a GPS flight plan won't work?
Essentially there must be a way for the copter to locate it's self in 3D space.
The plain accelerometers and gyros are not precise enough to hold position without some drift.
If you give us a better idea of what you want to do we might be able to give you some suggestions.