There is no danger if you pick someone reasonably healthy. If you actually follow through on your method of thinking, antibiotics are VASTLY more dangerous.
Yeah vitamin D is very important for gut health and health in general, but more vitamin D won't magically make new strains appear. Especially anaerobic strains that can't survive in oxygen. If you are given antibiotics as a child, it can kill off keystone strains like akkermansia which you NEED and will be lost forever unless you are given those strains again. Sun exposure doesn't create bacteria strains out of thin air (although I do wish it would). This is why probiotics are good for you but don't cure anything, because they contain aerobic strains but not anaerobic strains. You NEED FMT to fix gut issues. There is no other way.