Outside of the traditional levelling up, I'm not sure how else they could expand Ratchet, the series is at a good place with the A Crack In Time formula with the mini-planets, side quests and arena. At this point just more of that will be great for me, but it wouldn't grab new players. Maybe crazy vehicle customisation and upgrades to explore on planets for rarer items, like getting a crazy dune buggy and going to town, something like Jak 3 I think.
And heck no to random loot! Make it so the harder and rarer materials are always in the most challenging areas, but you're always guaranteed to get them. Make it so it's tough, but not Bullshit.