Hey there FOOF

Washout? No... you want upright and inverted flying qualities as similar as possible.
Symetrical section? Yes......., see above (Airfoil selection can alter the snap roll characteristics significantly. You don't want an airfoil that requires an AOA of 25 degrees before it stalls... this will result in snap rolls totally killing any speed or heading you may have had)
Dihedral? Maybe... Use dihedral to eliminate YAW induced roll. (Adverse roll, more dihedral, proverse roll, less dihedral)
Total fuselage length? 78.74" long. Minus 3.5" for spinner. Using "NORMAL" motors and construction techniques, set location of wing on remaining fuselage to achieve UNBALASTED CG.
Tailplane position? Located so that there is no (or minimized) YAW induced pitch. (Pitches down with rudder, lower stab, pitches up... raise stab)

Other things to think about.... How much vertical area below stab/fuselage (Subfin?), Thrust angles, taper ratios, control surface sizes, Exhaust system accomodation, Cooling inlet, cooling exhaust....... a system of compromises for sure!!

I would suggest that you first at least fly several of the current popular models. This will give you an idea of what there is to improve upon. With no concept of what you are trying to make better, you will more then likely just come up with what is already out there. If you want to just make a Pattern plane that looks cool.... then buy a wing and tail set and make a new fuselage.