It will be officially revealed on July 28th
Considering the name of Cooking Mama developers Cooking Mama Limited, it was obvious another game would hit a Nintendo console at some point in the future. According to Siliconera, that game is Cooking Mama 5 for the Nintendo 3DS, which is preparing for a winter release in Japan.
Considering the high rate of localization, it's probably best to just assume an overseas release is also in the cards (same with the upcoming Gardening Mama). Will Mama 5 be better than the first four Mamas? We'll know later this year.
Cooking Mama 5 [Siliconera]