The Xenoblade games can be pretty difficult, whether it's due to certain team combinations working better on some bosses compared to others (there's one towards the end of the game that's infamous for this), or due to level 60-80 monsters roaming around the world to mess up your day.
For Dragon Quest 11, I found that trying to fill out your Defeated Monster List and looking for rare variants of monsters helped with gaining levels, especially once you figure out which enemies will be grouped with Metal Slimes, Liquid Metal Slimes, etc. It also helps that, once your party dies, all of your remaining members jump in as back-up, so it couldn't hurt to get them leveled up as well.
I'm honestly fine with how most of the franchises are handled, though I feel like (Captain) Toad, Bandanna Waddle Dee, and Dixie Kong should be in. Zelda and Xenoblade should get at least one more character (though if assist trophies are ruled out, I'd have a tougher time with picking which Zelda character should be included - my first choice would be Midna), and I'd like to see another Wario character (ideally Captain Syrup, so we can get some material from Wario Land). Maybe Octoling from Splatoon, since they would be an easy echo fighter.
For third parties, the only series where I feel like they need another character are Final Fantasy (I'd pick Terra from FF6) and Sonic (torn between Tails, Knuckles, and Eggman). I'd like to see Jeanne from Bayonetta added as an echo, but I don't know if she's needed.
Unless you mean five new characters and five echoes in total - then it would be a lot harder for me to narrow the new characters down.
