Heihachi isn't boring. This is the same guy that threw his kid of a cliff, straps his rivals to space rockets, headbutts demons and androids and can survive explosion from close distance. He has so much character, that Pikachu acting like Heihachi in Pokken Tekken was instantly recognisable. Also just like Pikachu, his powers are somewhat electric based. Ryu is honestly one of the more interesting characters, he even has his own mechanics. And Heihachi might have the same. He won't have the same moves as Ryu. Anyone arguing that hasn't played both characters in both games yet.
In regards to Nightmare being a better choice, I'd love either one honestly, and I do think Nightmare is more visually appealing in terms of his possible special moves and overall look. Personality wise Heihachi wins imo.
I really want another Namco Bandai rep as it makes no sense to me to only receive one while they are co developping it. A fighting game character would be the best choice imo.