Starting with Rainbow Dragon in your hand (unless it was a free card, not taking up one of your opening hand) would be TERRIBLE. It's useless early game, unless we get Rainbow Neos, which I doubt. The best you could hope for is something like Para & Dox's Elements Unite!. Aside from that, I don't think there's much that could make Rainbow Dragon worthwhile.
TONS of cards that will likely never exist in Duel Links have voice lines. Jesse has a line for Crystal Abundance (which you can't even use without Destiny HERO - Diamond Dude) and quite a few anime-only cards, for example.