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A farm! Mareep, Ponyta, Miltank, maybe some Tauros, the new Kalos Pokemon Gogoat, and so forth. Growlithe can be the 'sheepdog' of the farm. There would also be a pond where Pokemon such as Ducklett/Swanna and other water Pokemon can enjoy themselves. With 50 acres I will have plenty of space for pastures for the Pokemon to hang out in.
Alternatively, a breeding center or a Pokemon Rescue.