Flying Crane
Sr. Grandmaster
Your best bet in my opinion is to study a variety of combat orientated arts and take what you like best from them, and make it your own.
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Was there something you liked from Budo Taijutsu that stood out to you? A particular variation of a gun disarm in Krav Maga? You get the point.
Personally, I have less faith in this approach. It is my opinion that different martial systems, if taught properly, have a very specific and underlying method for delivery of every technique. It isn't so much the technique itself, the specific movement or combination of movements, or the defensive action that matters so much as understanding and mastering the underlying method of delivery. Once you understand that underlying method, you can deliver any technique with devastation. If you only focus on the specific technique and movement and combination, then you are really looking a the system on a very superficial level.
Once you understand the underlying method, you will find that you do not need to cherry pick from various different arts. But it takes a very deliberate and focused study to make the breakthrough to be able to do this, and it takes an instructor who can bring you there. Not everyone out there is up to snuff.
Your suggestion raises a few very practical questions as well. You state that if there was something from budo taijutsu, or a gun disarm from krav maga, as examples, that you liked, then adapt that. Well, how long to you need to practice these systems in order to get those components that you specifically want? What if those schools do not exist in your area?
Personally, if I was running a school and someone showed up with the idea that he wanted me to teach him specific things about my art that he was going to just cherry pick and probably go on his way in short order, I'd show him to the door immediately. I have no interest in working with someone like that. With that kind of approach, he absolutely cannot learn what I have to offer, so I won't waste his time or mine.
I suggest one find a system that appeals to him, an instructor who is knowledgeable and capable, and study as deeply as he can. More answers will be found there, than by jumping from one thing to another like someone with ADHD.