I don't see anything wrong with starting your own system as long as the reasons for it and the principles upon which it is built, are valid. That is, if you can see a better way to build a mouse trap, then do it. But if it's just so you can call yourself "Grandmaster" or "Soke", for personal gratification that comes with wearing rank, then it's all wrong, and nothing you teach can make it right.
If the principles are based in technique and physics relating to body mechanics, then OK, but if it relies purely on physical strength and brute force then, No, it's all wrong.
Also, I think it is ok to teach what you know, even if it is a mixture of systems, so long as you are honest about where it comes from. That is, if you learnt your stuff by watching videos and reading books, and ended up with a fairly decent system of self defense, then admit you got it from books etc. Don't try telling people it's a 2000 year old system of martial arts handed down from father to son blah blah blah ala Frank Dux ad nauseum ad infinitum.
Above and beyond everything else be honest about it.
--Dave