I think nowadays, with digital distribution, Puyo Pop games could start being successful in the West for maybe the first time. The story mode, in my opinion, is a big factor - if you make the story entertaining enough, people might buy the game just to read it.
Anyway, something I'd like to see is a Puyo Pop game where about half of the characters come from other games - a Namco High sort of thing, if you will. Here are the characters that would be ideal, if the goal is total fanservice, and the fan in question is myself.

Note: The amount to which I've embellished the character bios ranges from "none" (Rulue) to "basically made up everything" (Corona).

Arle Nadja - A young but powerful girl who is very experienced with magic. She's fought several fearsome monsters and been in many dungeons but somehow remains incredibly approachable. (Madou Monogatari - 1990)
Billy Hatcher - This brave kid who saved an entire world of talking chickens by rolling eggs around and hatching them. It's harder than it sounds. (Billy Hatcher and the Giant Egg - 2003)
Corona - A 14-year-old who's been having memories in his sleep that he knows didn't happen, but they seem so real. The dreams show him talking to animals and becoming a formidable warrior. But how could that be? He's never even seen a sword... has he? (Crusader of Centy - 1994)
Draco Centauros - A green-haired dragon woman. She's not mean, but she is quite competitive, especially at Puyo. (Madou Monogatari - 1990)
Grounder - One of Robotnik's old robots. He's not very good at his normal job, but he is uniquely qualified for a Puyo Pop game. (Dr. Robotnik's Mean Bean Machine - 1993)
Marine the Raccoon - A determined girl from Southern Island who's hoping to explore the world. (Sonic Rush Adventure - 2007)
Ringo Andou - She does well at school and works at her parent's produce market. She keeps running into Arle, even though they're apparently from different dimensions. (Puyo Puyo 7 - 2009)
Rulue - A very good fighter who nonetheless is jealous of people who are skilled magic users. Rulue has a crush on a guy who has a crush on Arle, so that makes things weird. (Madou Monogatari - 1990)
Sig - For a kid with a big red claw for a hand, he does seem pretty normal. He likes bugs a lot. (Puyo Puyo Fever 2 - 2005)
Eddie "Skate" Hunter - He may look like just another radical 90s kid who enjoys skateboarding and wears his hat backwards, but he also helped his friends and his brother take down a criminal syndicate - twice. (Streets of Rage II - 1992)