You said it - this is a VERY subjective question.
In my opinion, if you are looking for a stable, slow flying airplane capable of carrying a GoPro, then the Bixler/Sky Surfer can't be beat. It is inexpensive, durable and has fantastic flight characteristics.
Think efficiency for your application in terms of aerodynamics. What is an efficient wing to meet your requirements? Two of the most important parameters from this standpoint are wing loading and aspect ratio. Wing loading is simply the weight of the aircraft and payload divided by wing area. Aspect ratio is the ratio of the wing length to wing chord (width from leading edge to trailing edge).
Slow flying airplanes typically have low wing loading and high aspect ratios. This equates to long, narrow wings. So, does this mean that longer is better? No, at least not to a point. At some point, the structural requirements of a looooong wing "outweigh" the wing loading requirement. Your question is, "what is the optimal wing design for a slow flying, stable aircraft capable of carrying a GoPro?" Um, I don't know.... There are more parameters to consider.
What flight times are you looking for? What size batteries are going to give you those flight times? That will impact wing loading and fuselage size.
What airfoil are you using? This is an often overlooked parameter of a scratch built RC aircraft. Most scratchbuilt RC planes use either a Clark Y airfoil or some type of KF airfoil. Both work, but neither is ideal for flight duration. Plus, the Clark Y creates a strong, pitch down moment requiring either a longer fuselage or larger horizontal stabilizer than possibly necessary. Now, I've been called an "Airfoil Diva," and many will argue the finer points of the necessity of airfoil design.
Now that I've said all that, think long, thin wings. You can taper them from the wing root to tip or just use a constant wing chord. A wingspan of 1400mm to 1800mm will work nicely for carrying a GoPro at a stable, slow speed. Think wing loading in the neighborhood of 10 to 12 ounces per square foot. Not sure what airfoil I'd go with, off the top of my head. Something close to but not quite symmetrical.
Good luck!