Probably the biggest issue in building with alternative materials is the adhesives.
Glues such as Polyurethane Gorilla glue can add multiple ounces because they are the same final density as epoxy. They require the same usage tricks to keep the weight down- small dabs instead of globs. If possible apply it in a thin layer.
White glue and Carpenter's glue cure by evaporation. Make sure any joint has open air or wood to transfer the water out. If not, these glues can literally take years to dry as they mildew.
Cyano-Acrylate- aka Hot Stuff(tm) cures to type of solid acrylic acid polymer. If you've ever picked up a piece of plexiglas it is lighter than glass, but much denser than balsa. Use the absolute minimums. Plus it is a very expensive substitute for dope.
Contact cements- 3M 77 is common. It can be effective with very light coats. Makes iron-on glass reinforcing, do it yourself iron on film.
Lite Ply saves about 1/2 the weight of same thickness birch ply. It's not terribly rugged because it is brittle. But it's easy to glue back together. The RC'er use it(ARF's and kits') by cutting out lots of panels out of, say, a fuselage side. Using a very open structure like that, with proper design, is plenty strong for ordinary flying. It does crack here and there in hard landings though, which can be hard to spot.
Carbon matte, carbon fiber cloth- all require careful molding, preferably vacuum molding and/or application to keep the weight down. Excess/Meager resin can cause spots that will fail.
There are lots of "tricks" building "foam" wings for various planes. If you want to learn see how the design/construction has changed over the years. The most common method changed from16/-3/32 balsa epoxied to blue styrofoam. It pretty quickly change to the white bead board which about half the density and comes all the way down to .8lb/cuft.
The latest tricks are using the foam as an inner mold and then removing all/most of it after the outer surface has healed.
There are a lot of different ways of mounting landing gear, retracts, internal fittings, hinges, etc. Some are over kill, others fragile.