To entire these new randomly generated dimensions a den similar to to those found in swsh. You right click them and are presented with info on the dungeon, with a silluette of the final mon and types of wild mon to be found inside. As well as the star rating (1-5) which effects the length of the dungeon and level of opposing pokemon.
Here the random generation comes in, where a lot can be drawn from mystery dungeon. There should be a verity of pre generated rooms, which can randomly paired with aggressive pokemon. Leading to a large boss chamber where you battle a large dynamax mon.
These new dungeons come with a lot of challange so the rewards should match it. Besides loot the first reward a player receives for beating these dungeons is the dynamax band, which allows the players to dynamax their own mon while in available areas.
P.S. this same concept of a dungeon mini-dimension could be used for an Elite 4 sub dimension where a full badge case acts as a portal.