By my idea of the "tag" system (I call Swap), a duo (two characters) would be allowed and it would work more like Old-Young Zeku in SFV or Melee's Zelda-Sheik than MvC types.
I'd try out a lot of combinations with Wii Fit Trainer, but would probably main Wii Fit Trainer (Classic Moveset) + Wii Fit Trainer (Extended Move set).
I created the SFV animated gif becuase it shows what a Swap would look like and what Directional Swap Cancelling looks like. You just have to imagine the player assigned the "Spawn Point" (young Zeku in air) to a command (Direction + Swap Button) and see this as the critical point that allowed him to complete the air juggle off "swapping" in Young Zeku from Old Zeku. See link below for details.
https://smashboards.com/threads/wishlist-of-stuff-for-smash-bros-for-switch.446712/post-22038808
People may think I'm coo-coo with this concept, but there is nothing foreign to Smash in anything I described in that post. EVEN what players could dictate with Greninja could be seen as a far more limited version of Directional Swap Cancel.
We've been talking about the components of this stuff for years. It's just that I've found a system I'm pleased to suggest.
Note that I have considered potential solutions for keeping Directional Swap Cancelling from being absolutely broken. As with everything else, that is obviously VERY important. I didn't want to go into these in my overview picture this time, but stuff like Heavy Duty Knockback Scaling all the way up to this concept of a Rubber Band Blast Zone ("can't kill with this unless enemy is over 110%", for example) are good potential solutions, I think.
Directional Swap Cancel is a potential solution to a problem of how to bring out the best in Swaps. While being able to select any second character from the roster or even the extended version of a character, we even bring in a reformed version of Custom Specials. HOWEVER, the "Custom" in "Custom Specials" gets topped off with Directional Swap Cancel, where another level of "move customization" occurs. What DSC allows for does more justice to the word "Custom" than was there in Smash 4's Custom Specials. We're not talking the actual moves the characters were programmed to do, but rather the combos and shenanigans the player is FREE to imagine, test, and actually pre-assign critical spawn points that make it even possible to do them in an actual match. You are basically creating your own personal deck of cards to bring to the fight. The system is ABSURDLY open-ended and all players do is set up multiple "teleport" points that make the magic they designed possible! Imagine what that does for gameplay. Imagine what it does for SPECTATING high level matches. Shoot, there is a way for players to even put their SIGNATURE on stuff as ORIGINATOR by the game assessing certain values and comparing with what has been uploaded.
-checks all relevant conditions (especially duo used)
-checks inputs that preceded the Directional Swap Cancel
-checks inputs and their timings that followed the Directional Swap Cancel
-checks damage and/or anything that took hits
-checks timings
-checks whatever is necessary to make a thorough comparison to Custom Swap Combos & Shenanigans already uploaded
So the side point here is through this, the game can determine what is original or not and players can actually lay claim to stuff they pioneered in the way of combos or shenanigans.
Wonderful potential, I tell you! I wish more people were up on it because then we could go from talking about our teams to describing Directional Swap Combos according to current and imagined movesets. For starters, we could look at Team Combos from 2 vs 2 tournaments for inspiration. Those would be possible (and in the manual way we know of Smash, not that canned/automatic stuff) with Directional Swap Cancelling.