To put it mildly, Nintendo's Mario sports titles have been divisive. Starting with Mario Tennis Aces in 2018, Nintendo seems to release them in an unfinished state, without enough content to satisfy fans at the launch, and then refuses to offer any more content after a few months, as fans are lucky. This is the most recent, and probably the most blatant, example of this problem.
Mario Strikers: Battle League was immediately chastised by both reviewers and loyal Nintendo fans for having little content at its release. However, Nintendo has stated that it will continue to support Mario Strikers: Battle League until the game's release, and that one free DLC has already been released.
Mario Strikers: Battle League DLC Wishlist
Both Shy Guy and Daisy were brought to the game as playable characters in Nintendo's first Mario Strikers: Battle League DLC, which was released just a few weeks ago. A new type of gear for all players was added, as well as a brand new Stadium, named Desert Ruin, which is a solid first step, even if it does feel like this content should have been included in the game.
Future Mario Strikers: Battle League updates should follow the same general formula as this first pack. Each new update should include at least one new playable character, with their own set of distinctive animations and stats. For example, Kirby and the Splatoon Inklings could be added if Nintendo wanted to go the extra mile.
Any future Mario Strikers: Battle League DLC should include new stadiums, most of which are very generic, and that they don't all fit into the Mario universe. It may be worthwhile to recreate some of the older games' stadiums, as Mario Kart 8 Deluxe's Booster Course Pass is doing.
The most important expectation for Mario Strikers: Battle League DLC is that it be released consistently and regularly. According to Nintendo's general roadmap, there will be two more DLC releases before the end of the year. However, this amount of updates fits in with the recent Mario sports titles trend, and it may happen that after those two waves, post-launch support stops altogether.
Mario Strikers: Battle League is now available on Nintendo Switch.