As for RH: it’s an interesting theory. But, who is to say that RH didn’t go all “evil” and bad when Marion died and maybe both he and his daughter Tamara joined THO.
Turning RH into one of the evil guys would be a ballsy move. The character is iconic because he’s one of the good guys.
I can’t see a Robin Hood/Regina romance because Robin’s true love is Maid Marian and they have already been established as characters so Regina can’t be Marian in the way that Rumple is the beast.
The Regina/Robin thing is dependant on Marian being dead, and these two being each other’s second shot at love.
One of things that I’d like about it if they used RH as both Tamara’s father and Regina’s future partner is that they’d be able to cover two necessary storylines without having to bring in new characters we haven’t met yet. If there’s one thing the majority of the fandom can generally agree on, it’s the fewer new characters they add into the mix, the better.