1(Shadowhunters)

An Au where Lydia knows that going on with the wedding wouldn't do any good to Alec who she came to see as a friend so after hearing Izzy trying to convince Alec to follow his heart and go to Magnus, Lydia decides to go to Magnus herself.

Once Magnus agrees to talk to her, Lydia explains that while she likes Alec it is as a friend and she wishes to see him happy but he is set in following what his parents want so he can finally have his dream of being the official HOTI(Head of the Institute) of New York come true.

While Magnus does no understand why Lydia tells him that, she explain how she won't stop the wedding but she wants him to take her place as Alec's spouse.

They would both be under a glamour(Lydia as some random shadowhunter and Magnus as Lydia) till the ceremony began as it would be revealed that Magnus was the 'bride' and Lydia was among those invited.

Of course that while unsure(because Magnus never thought of marrying, he and Alec hadn't even gone to an actual date, Alec is mortal, how his past relationship ended, etc), after thinking for a day he ends up agreeing with Lydia so they ash up their plan better(Magnus approaches Tessa to have Jem to help them, make sure there wouldn't be any problems by having some friends of Lydia who are in high positions support Alec and Magnus marriage, have the help of Magnus closest friends, etc).

Anything else would be up to the writer including everyone's reaction to Magnus being revealed as the 'bride', how things came to be, if Robert and Maryse are decent parents, etc.

I know this idea makes no sense but I have read a few fics about how the wedding scene could have gone(both the wedding being ended before they are at the altar or similar to canon) and I always wondered what would happen if Lydia was the one to take action to make sure Alec would be happy and be with someone he likes without losing the things he fought so hard for.