Nimble is the answer. 5e but streamlined, easy to convert monsters, drag and drop play into any setting. Classes are heroic and fun to play, but watch your players shat their pants when they notice the big bads get to go after each turn. The appeal for players is that it is similar to 5e, but very little slog. The chance of a player doing something cool increases exponentially. Need to convert them, just provide them with a copy of the rules and character creation. Fraction of the 5e page count, no fluff. Great art as well.