The new character combo we need is DK and Diddy, search your feelings for you know it to be true, and it'd be a nice change of scenery as well.
Mario and Luigi can take a break, they've earned it after four games.
How are any of you finding this game difficult in the slightest?
It might be the toughest game in the series when I think about it, not by much but I found some of the late game enemies to have some of the more trickier to evade attacks of any in the series as well as a few that hit like trucks. But the game gives you plenty of tools to work in your favour, I remember at one point in the game Mario had boots or something that gave him health from damaged enemies along with a health giving bracelet making him a pure regenerating machine, especially broken in the dream world.
Then you've got the badge mechanics and the class up bonuses, in ways it's easier but as I said, some enemies hit hard, possibly to compensate for all these little extras.
But really M&L games are only as hard as mastering the enemies attack pattern, once you've mastered that then there's not much to worry about.