I am not sure about a golden age. There is a lot of R&D in the "cheap" end but a suprisingly small amount in touring aircraft. Cirrus & Diamond are now old hat (and unless you particularly want a diesel irrespective of reliability, a TB20 beats both of them), the Lancair is no good for grass so very limited for Europe, there is the Czech VUT-100 which is very much like a TB20 and which is slowly coming along, and...?
I confess I was thinking of the PFA end of the market, the Ban-bi, RV series and Silence Twister (although that's a single seater). The way I think about it, we've gone from high weight, high drag, high power (relatively speaking) to low drag, weight & power in the space of 20 years - while increasing speed! The TB was an early attempt to reduce all these variables, and it worked well for its time. Now that GAMI & Deakin have shown the way, it's a far more capable aircraft, as are the Bonanzas. But problems remain. Not many pilots know or care about leaning LOP, min drag, fuel flowmeters, best range, EGT, CHT, oxypulsimeters etc. Even if the manufacturers developed the perfect airframe, we're still limited by the engine, and by pilot ignorance.
So we take the engine out of the loop by introducing true FADEC - the Rotax V300 springs to mind - and we marry it to a lowish drag airframe like the Bonanza. That's the near future. I'm not convinced that diesel will saturate the market, it's fairly unreliable at the moment, but will improve hugely, and Jet A1 is freely available. Solar and regenerative motorgliders will become huge sellers, but as for the fate of AVGAS - I don't think it'll be around for that much longer - i'd give it 7 - 10 years. And we'll be better off without it.