I've actually thought about that quite a bit, and naturally it incorporates some elements from my favourite games.
You'd have a few planets (7-10). Each area (planet) would be open, but would of course be enclosed to only that planet. And no, the areas wouldn't be planet-sized. Each area would be rather big, but not too big. Think big enough that you could explore for a while, but not so big that you can't explore it all.
Gameplay would be like Ratchet & Clank (Everyone saw that coming, right? 
) but focus a bit more on the platforming. Your weapons upgrade, but in this game, other things, like your hoverboots and your boost jump would also be upgraded, so you add a few more RPG elements.
Have some bigger side questlines, and a mainquestline, just like in your normal WRPG, and of course you have the challenge mode.
So basically, what I'd make is an R&C with more exploring and more RPG elemts xD
