Young Macintosh I like how some writers put him with Merida sharing a few of the same talents and hobbies while on adventure. Her sequel could sure introduce some of the fantasy from her culture, especially magical horse stuff with tapestries it wouldn't have to be a magical horse on adventure it could be a made up celebration because the unicorn's on Scotland's flag and it's not the only magical horse from Northern Europe
If we wanted this introducing a magical thing from Scotland and neighbors like Ireland we could let Merida and Young Macintosh have the adventure meeting a Kelpie but balanced like Merida's scenes of seeing a bear. These two seeing the kelpie would let them both show their talents, they could have scenes of trying to save Angus or a wild pony cousin from the Kelpie. The kelpie would be even a bit cooler than the bear scenes are in the first film or if the sequel wanted to it could show both a bear and the kelpie. If both are shown in the sequel it could be something like her brothers getting the same food so they turn into little bear cubs again and can swim the river to help Merida save the horse or pony she trying to rescue from the Kelpie. I liked the idea of Macintosh coming when it was one of the Celtic Festivals this is the type of idea some writers showed. Tapestries of unicorns in the castle and the Kelpie met in the water, this way the film's showing as much horse stuff as the first film showed bear things.
This is the new idea I can come up with for an all new adventure for this surprisingly brave princess. Festival stuff and magical scenes throughout the film again.