2022 actually (Halifax hosted this year).
I imagine the QMJHL would prefer a Quebec based team to host the next time it's their league's turn to host the Memorial Cup. Gatineau would certainly be a strong contender.
So, the first possible year for the Avenir Centre to host the Memorial Cup would be 2025. Moncton however would be facing strong competition from Saint John (a city which has never hosted the Memorial Cup). I we don't host the cup in 2025, it likely won't happen until 2031.