I recommend thinking that taking 10 on someone is like taking 1,000.
Enemies from Final Fantasy wouldn't work, they'd look too serious. Having Mario and Cloud fight side-by-side wouldn't be unheard of - we've seen stranger teamups in Smash Bros. Brawl - but again, odd considering how serious Cloud is compared to, say, Mallow.
Yeah, I don't think they should include Final Fantasy characters.
There's really only one reason I skipped out on Paper Mario and its sequel: how everything's dumbed down to low HP levels. How you can never seem to make an attack that's more than 10HP. Made me felt eternally weak, made me felt like my options in battle were restricted,it was so different from everything I was used to - and not in a good way. I want the triple-digit numbers! I was also a bit ticked off that in Super Mario RPG, the highest level you could reach was 30 (but then, you're so powerful by that point that going beyond doesn't matter).
To tell the truth, Super Mario RPG Legend of The Seven Stars has a lot less options than Paper Mario or its sequel. Most special attacks usually consisted of "Tap A as fast as you can!" or "Circle the D-Pad as much as you can!" However, special attacks in the Paper Mario games have more diversity, and often have a greater result. The badge system also works well. Restricting the amount of Attack and Defense you can have (except in Super Paper Mario; that doesn't really restrict Attack, or have badges) works out fairly well with the stats of enemies and bosses.
Sorry for this off-topicness, but these are my current stats in Super Paper Mario: 350 HP, 80 Attack. I know some of you know I'm going for max stats, so wish me luck!