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I know we like to joke about Sherliam already being married, but in your opinion, who would propose? I feel like Sherlock would be the obvious choice, him being infamously impulsive and all, but I think that Liam proposing would be the cutest thing ever. Just imagine Sherly's reaction.
On another note. How about a modern AU where Sherlock gets pissy because John didn't tell him he was going to propose to Mary and storming out to buy a ring for Liam, proposing to him that same day and planning a small wedding and not inviting John. Liam goes along with it because he thinks it's funny, and Louis wants to stab Sherlock more than ever. After a week of apologizing and being ignored, John finally gets Sherlock to come back to their shared flat, only for Sherlock to immediately introduce him to his new husband. They show him wedding pictures with a cardboard cutout of John as Sherlock's best man. They hold a larger ceremony later that year that John is allowed to attend.