There' a problem with that though. The bard has a
niche (
a couple good ones actually), it just fills them badly in 5e because it would be overpowered if those niche roles were powerful enough to be called out rather than glossed over as mere "bells & whistles" alongside "modest to good firepower", "modest to good" defense, & "modest to good" HP The same applies to most of the thirteen classes in fact. 5e said "screw that" and declared that only very specific niches were valid so that every class needs to be "modest to good" at worst in them & as a result any other niche they had needed to be dialed back from great towards "modest" because "good" would be too powerful with "modest to good" everything else.
Every class should be able to meaningfully contribute to every aspect of the game that is considered a vital part of play. In 5e, this has been helpfully defined for us: the three "pillars," combat, exploration, and socialization. These are the things the game designers have explicitly called out as what D&D 5e is "for" in some sense. Classes not designed to meaningfully contribute to any one or more of these pillars should not exist. Period. If the designers wish to make a game that is about other things, that's totally fine--but they should then discuss what this new game is itself about, so that we can know what its player options should do.
Your definitions are unlikely at best.
... I'm sorry, what? I'm literally using the words in their common meanings. A cooperative game is one where multiple players coordinate with one another in order to succeed (whatever the metric of success is for a given game). An asymmetrical game is one where different players have genuinely different resources and contributions, such that it is not possible for player A to contribute the way player B does (and, usually, vice-versa; it's generally unwise to make an asymmetrical game where one player can do everything another can and also more things too, not ALWAYS unwise but generally.) Baseball, for example, is cooperative (within a team), competitive (between teams), and partially asymmetrical (different players have completely different and distinct roles while playing "defense," such as pitcher or outfielder, but just one role while playing "offense," batter). D&D has a referee, the DM, who is not "playing" the game in the usual sense (hence why her characters are
non-player characters), just as baseball has an umpire who is not "playing" either. So if we focus on just the "player characters," the game is symmetrical: each player is offered an identical selection of options for approaching the game, which the text of the game treats as neither stronger nor weaker, just different; and each player is responsible for the same thing in terms of the success metric, rolling high numbers on dice (or obviating the need for dice) in each of three important areas, explicitly called "exploration," "socialization," and "combat." This has been the case (other than the official and explicit callout of the three "pillars") in, as far as I'm aware, every edition since
at the very least 3rd, and possibly much earlier (I have not read the 2e nor 1e PHBs in full like I have the 3e, 4e, or 5e ones so I hesitate to make sweeping statements about their contents.)
What version of D&D, at least since 3e, and (unless I'm mistaken!) possibly 2e, tells the players, "This class is just better than that class"? Which version of D&D tells its players that it is not a cooperative game?