My favorite Devil Fruit is a toss-up between the Rumble-Rumble Fruit and (after the episode before this newest one) the Op-Op Fruit. Lightning is just awesome, with how flashy and powerful it can be, and Law's use of the Op-Op Fruit was really cool to see. The Magnet-Magnet Fruit has turned out to be a lot cooler than I would have expected, too.

As for what powers I'd give my character, I originally wrote Sol and Luna as analogs for Apollo and Artemis, so I gave Sol fire powers and Luna control over ice. As such, I'd probably give them the Flame-Flame and Ice-Ice Fruits, respectively. The Phoenix Fruit for Sol and maybe Okuchi no Okami for Luna could be good alternatives, as well.