Mario Strikers: Battle League will get free DLC content updates that include new characters. A demo is available to try now through Nintendo Switch Online.
Free Update for the Mario Strikers: Battle League Game - Play Nintendo
Mario Strikers Battle League adds Diddy Kong and Pauline as playable characters
Mario Strikers: Battle League
Mario Golf: Super Rush DLC Roadmap (2021): Free characters and courses - GameRevolution
Predicting Mario Strikers Battle League's Final DLC Characters
Free Update for the Mario Strikers: Battle League Game - Play Nintendo
Mario Strikers Battle League datamine reveals 10 DLC characters coming - Dexerto
Mario Strikers: Battle League - All Hyper Strikes (DLC Included)
Mario Strikers: Battle League - All Hyper Strikes (DLC Included)