After a certain amount of years of training can we really get any bigger? Can we gain muscular size? Will Our body look any differant? My opinion, I don't think a natural person can build more muscle or get bigger without gaining fat. After so many years of training and being older I believe You probably reached what You're going to be able to do genetically.
Will Your body look any differant by using differant training types? Again just my opinion, depends on alot of factors. Some don't want to hear this but diet is the most important factor. If You have a bad diet nothing is going to help. But I have experienced not changing my diet and doing a couple differant training types. One may make me look skinny and another thicker. Am I right I don't know, not an expert or scientist.
Will self resistance, VRT, isometric, calisthenics, ect... make You look more muscular? I say if You enjoy it, it makes You feel good and healthier who cares. Honestlly I believe they can with some effort. If You look at a guy like Bob who has gone through alot with his sickness, this guy has done a great job. He enjoys what he does, looks strong as a bull and he looks pretty solid. Just enjoy what You do, that's all that counts.
My buddy that was a weightlifter and injured, now uses a 5 foot red oak staff as a barbell and loves Atlas Isometric Weightlifting exercises (AKA VRT) and it is working for him! He is looking great, plus reports his friend is using Isometric Weightlifting (VRT) with "The Action Hero Body" training book by Jorgen De Mey and also doing great. He uses Isometrics for the weightlifting and does the Calisthenics.