I reeeeally can't see them giving the shield to Sharon. As much as I love her, she was such a small part of Winter Soldier's plot, and casual fans probably wouldn't even recognise who she is because she doesn't (in the films) have a cool superhero name or uniform. I'd imagine that's a red herring, like you say, or somebody's hopeful speculation being repeated so much it becomes a rumour.
I think it'll be Bucky, honestly. SebStan's got a pretty long contract, if I remember, and he's been an integral part of Steve's story since The First Avenger. I'd love it to be Sam, but it actually makes sense in terms of the movies for it to be Bucky. In the comics, there are countless legacy characters and people switching superhero names, but that's not a big thing in the MCU. (The closest thing would be Scott taking on the Ant-Man mantle, and the general public aren't even supposed to know Ant-Man existed.) Sam has his own strong identity as the Falcon in the movies, but after the events of Winter Soldier, Bucky doesn't have his own identity as a superhero (and is still struggling with his personal identity, I'd imagine). Having him become Cap gives him that identity.
Re: Interesting patterns you've noticed in fanfic
I think it'll be Bucky, honestly. SebStan's got a pretty long contract, if I remember, and he's been an integral part of Steve's story since The First Avenger. I'd love it to be Sam, but it actually makes sense in terms of the movies for it to be Bucky. In the comics, there are countless legacy characters and people switching superhero names, but that's not a big thing in the MCU. (The closest thing would be Scott taking on the Ant-Man mantle, and the general public aren't even supposed to know Ant-Man existed.) Sam has his own strong identity as the Falcon in the movies, but after the events of Winter Soldier, Bucky doesn't have his own identity as a superhero (and is still struggling with his personal identity, I'd imagine). Having him become Cap gives him that identity.