Coin lockers at train stations
Almost every JR and metro station in Japan has coin-operated lockers. Small (¥300-400): fits a backpack. Medium (¥500-600): fits a carry-on suitcase. Large (¥700-800): fits a full-size suitcase. These are your primary option. Use lockers at the station nearest to the stadium.
Stadium-specific options
Tokyo Dome
Suidobashi Station and Korakuen Station both have lockers. Tokyo Dome City also has luggage storage. Plenty of options.
Jingu Stadium
Gaienmae Station has lockers but they fill up on game days. Arrive early or use Shibuya Station lockers (one stop away on the Ginza Line).
ES CON Field
Shin-Sapporo Station has large lockers. The F Village complex around the stadium also has storage options.
Koshien Stadium
Koshien Station lockers are limited and fill up fast on game days. Use Umeda/Osaka Station lockers before heading to the game.
Luggage delivery services
Yamato Transport (Kuroneko) and Sagawa Express can ship luggage between hotels for ¥2,000-3,000 per bag. Send your suitcase to your next hotel in the morning, go to the game with just a day bag, and your luggage is waiting when you arrive.