Seems reasonable to have some ARAM after the majority of Missions force players to play Summoner's Rift.
Seriously though, I'm okay with Missions requiring ARAM (or preferably Twisted Treeline) as a counterbalance to Missions requiring SR. Complaints about "Riot shouldn't force me to play ARAM" come up every time there's a relevant Mission, but few people seem to recognize the opposite perspective of "Riot shouldn't force me to play SR." Just as TC doesn't want to have to play a game mode he dislikes just to get rewards, I don't want to have to play a game mode I dislike just to get rewards. Missions that require SR games (e.g. "Kill Dragon/Baron/Scuttle." "Place a ward." "Have X Vision Score.") are no better than Missions that require ARAM games. Ideally, Riot would offer alternate objectives more often rather than restricting Missions to a particular map.
With that said, I'm okay with the occasional map-specific Mission as a way to promote that map. Worlds Missions focusing on SR? Sure. Snowdown Missions focusing on ARAM? Alright. It's just annoying when map restrictions happen regularly and without reason (e.g. Ranked TT being excluded from the Ranked 2019 Missions).