Hmm...
Let's actually work with this a bit. My idea was that you could go to a "shop" in-game to buy random items: trophies, items for Adventure Mode, CDs, etc. One of the parts of the shop was a card shop. You could buy a pack of cards that would give you maybe 5 random cards and 1 guaranteed "rare" or "super-rare" card. Cards are added to your collection like trophies, and can be viewed at any time in the card album. You can check out cool official art, new art made exclusively for the card, and read up on background info, attacks, and stats.
I figure the cards would come in a few different varieties: Character, Item, Energy, and Boost.
Item cards are just that: items. You can play them to use that item with your character and deal whatever effect they grant. When swiping them for Adventure Mode, they give you whatever item they hold or, at the very least, an effect that is similar to it.
Energy is required to power attacks. Say, for example, the Fireball requires 1 Energy. If you don't have the energy, you don't have a Fireball. In Adventure Mode, Energy cards restore health.
Boost would be things like "Up Speed," "Plus Power," etc. These work in Adventure Mode exactly like the card game.
Character cards are the most interesting, of course. There are Common, Uncommon, Rare, and Super-Rare cards. For characters, this translates to a few different things. Character cards have classes, which are sort of analogous to the rarities (at least, because the stronger characters are of course rarer).
Broken down, Character cards go:
-Monster: the cheapest and typically weakest cards. These include plenty of generics such as Shy Guy, Metroid, Moblin, or Eggplant Wizard.
-Assist: cheap "humanoid" or "sidekick" characters, basically Assist Trophies. These are stronger than monsters and have slightly higher costs. Examples include Toad, Mr. Resetti, Kururin, etc.
-Ally: more experienced and costly characters, these are "Secondary" characters but not necessarily "assistants and sidekicks." Examples would include Samurai Goroh, Mona, Chozo, Amy Rose, etc.
-Hero/Villain: the main players! There are Common Hero/Villain cards, but many of the better ones are placed in Rare or Super-Rare. These include the likes of Mario, Ganon, Zelda, and all the others. All playable characters are either Hero or Villain cards, and there are many others who are not playable that appear here anyways (examples: Little Mac, Takamaru, Andross)
-Legend: These are "alternate" forms for many of the Hero/Villain cards, as well as "end bosses" from the Smash series. So here you would find Master Hand, Crazy Hand, and Tabuu, but also "transformations" of the Heroes and Villains. Fierce Deity Link, Giga Bowser, Star Mario, Metal Mario, Hero of Time Link, etc. For ever PC, there is at least one "Legendary ____" card that shows the character in their fittest, most original form.
Characters are repeated too, Pokemon TCG style. So you can get Mario, Shadow Mario, Metal Mario, Fire Mario, Tanooki Mario, etc. etc. In a few cases, these can be played as their own characters. In others, however, you have to either use a Boost/Item card to "evolve" the card OR cannot use it if a another copy of that character is already available.
In Adventure Mode, Character Cards have varying effects. Some summon an Assist, others boost the character if you're the same one as the card, others transform you into the character, and finally some just give you extra abilities. For example, Hammer Bro. appears as an Assist; if you play Mario while you swipe any Mario card, he gets stronger; if you play Zelda and swipe the Mario card you turn into Mario temporarily; and if you play as Mario and swipe the Lip card you summon a large Garbage Block to flatten foes.
....in case any of you are curious, NO, I've had this idea for a while, so I was able to somewhat develop it. I'm not that much of a loser that I came up with it on the spot for no good reason >
