In some of the original drafts of Brave, Merida fell in love with Young MacGuffin and eventually chose him out of her three suitors. Though it was decided that Merida should remain single at the end of her film, it's still hinted that Young MacGuffin has a crush on her. For this reason, Meriguffin is the most popular ship in the Brave fandom.
Merida and Young MacGuffin's relationship can vary, depending on the writer. They can be polar opposites, with MacGuffin being sweet and gentle to counter Merida's hot-bloodedness and Merida being outgoing and talkative to counter his shyness. This dynamic is very reminiscent of the canon relationship between Merida's parents.
On the other hand, of MacGuffin's three lines in the movie, two are somewhat sarcastic and meant as joking jibes against Wee Dingwall and Fergus. This indicates that, despite his quiet personality, Young MacGuffin actually has a pretty sharp wit, something he would have in common with Merida. He is also shown to be a fierce fighter when he gets down to it, being at the front of the hunting party at the end of the film, and he wasn't too excited about marrying a girl who didn't know him, so he and Merida are a fairly good match personality-wise.
Young MacGuffin isn't usually featured in RotBTD fanfics, likely because he is a minor character. He is also not too interesting as a rival for Merida's affections, as he likely wouldn't put up too much of a fuss if Merida decided to marry somebody else unless the relationship was bad for Merida in some way. Therefore, he is more likely to be featured in a War AU, where a crossover pairing might be seen as Merida consorting with the enemy.
Note: Please only add ones that are part of the RotBTD fandom