Job #01: The ultimate need for a cast like this, more than anything, is sheer expansiveness... but with uniqueness, and appeal. Plenty of characters (or creatures, or objects, or... you get the idea!) to pick from always lends to these kinds of games having great variety, and it's telling that one of the most popular trading card games out there (
Yu-Gi-Oh!) has anything from the infamous Pot of Greed all the way up to a quartet of stickmen on paper planes ripped directly from the
Parodius series. (Yeah, really.) But what's expansive enough to really capture people's interest?
Music.
Okay, hear me out.
This might seem like an odd submission at a glance. Where would the interest in music come from in a very non-musical genre like trading card games? Well... it isn't lost on me that there's a large amount of appeal that comes from "gijinka", being anthropomorphization of animals, objects, and so forth - just look at the art constantly coming out of the
Pokémon community, for example, or the appeal of highly-stylised interpretations of historical figures in something like the
Fate series. Hell, SEGA had a really neat project going on fairly recently with their
404 GAME RE:SET, featuring gijinka interpretations of their (and other companies'!) arcade titles. (I nearly went for arcade titles to be honest, but this feels likely to have more of a mainstream appeal?)
So, in this case, I'm suggesting that our "cast of characters" would be made up of characters who are interpretations of different musical releases, named after (and inspired by) a wide variety of artists, albums, and songs, but specifically not being "characters from music". (For instance, a character named IGOR wouldn't literally be Tyler the Creator.) In a sense, you could also have characters take influence from other related elements, like album art and individual lyrics! (Think like how this is sometimes used for Stand design in
JoJo's Bizarre Adventure.)
Again, I know it's a little out there, but I think this works well for a number of reasons. It grants us the unique opportunity to cater to essentially endless tastes, all while helpfully being able to categorise our cast by metrics like genre, era, and/or label - while I think it'd also get the creative juices flowing in terms of how people would try and capture their characters, and then slot those characters into a system that feels fitting for them. (And, we can use album art to indicate character designs... while expanding everyone's musical palette in the process!)
Beyond that, it also gives us the opportunity for cool mechanics and naming schemes; like your hand being named your Mix, or being able to Mashup cards to fuse their effects (as an example.) There's plenty of influence from music that we can take when considering mechanics.
And, like, let's be honest... who wouldn't want to see some utterly wild deck headed up by a silly mix of something like
Giant Steps,
Temporary Secretary, and Grammy-Award Winning 1999 Hit
Smooth By Santana Feat. Rob Thomas of Matchbox Twenty?