This is based of of the cartoons not movies or comics
When referring to Gaang it will equal all of Aang's era team avatar aka the Krew is Korra's era
UA is a collage so ALL underage characters will be aged up except Eri
While I Love Azula and you can change this I will be writing this idea based off a adopted reincarnated daughter of her in Korra's era so she will be in the Krew Azula knows Mc is reincarnated but not from another world The Mother of faces visited Azula when she adopted Mc when she was 5. Mc gets a spirit companion a fox.
Double element Mc because of reincarnation so she will be semi favored by the spirits fire and water so she will have white hair blue eyes with amber flecks and pale skin
Love interest is a must though you can make it a harem the love interest is Ryuko Tatsumi aka Dragon lady. Mc and Ryuko must adopt Eri
For the Gaang's era right after the comet for the Krew it is right after Korra got better
The Mc's reincarnation will be dramatic and traumatic for she will have died by fire and drowning I will not go into it but it was a cruise boat accident the spirits of the bender world gave her both elements in hopes of helping her though she will favor her water bending until she officially joins the Krew
So the Gaang and The Krew are transported to the very beginning of MHA in UA by a trickster spirit the spirit didn't mean to send them to another world but to the spirit realm on orders of Vatu in hopes of tricking someone to let him out it obviously didn't work. When everyone lands they are very obviously confused and weary after all the Gaang were just fighting and the Krew were planning after calling and checking on their respective groups they finally noticed the extra voices and looked over and because I like the head cannon of Aang being the last avatar of his cycle and Korra being the new start of hers while the two recognize each other they don't really feel connected so while everyone is confused and talking our beloved Mc is just blankly staring at a young Azula 😮😵😱 Asami and Ty lee are the only ones to notice when Ty lee ask Mc why she is staring at Azula Mc monotony says she is her mom cue confused panic by all except Azula and Mc cause some how Azula knows she meant adopted and just goes yeah I can see it cause of Mc's demeanor cue more panic(we all know Azula is internally freaking out till she realizes that she has someone who genuinely loves her then it is internally celebrating after the doubt passes). After all the freaking out Toph and Bolin ask where they are and in comes Nezu and the teachers the two groups must get in fighting stances and Azula must get in front of Mc to the internal shock of everyone and Azula but not the Mc cause the Gaang know her and the Krew have heard about her. Nezu ask who they are and how they got here in comes the Mc's spirit companion it answers questions of who they are how there of different timelines of the same world how a trickster spirit sent them there how because of it the two worlds are combining and the bender worlds timelines are also combining and explicitly states that when the time comes for the avatars to do their duty the humans of the MHA world are not allowed to act unless the avatar invites them to join and that the spirits will be blocking them. This can happen during the ten month time skip or during the school year if it happened during the time skip the two groups will have to learn about the MHA world during the school year as ambassador's for the four nations.
For the UA collage changes it will be like the typical collage au's already done but for the work studies its more of an interview for the Hero's who are mentoring so face to face meeting to find who is most compatible and will be genuinely helpful and the first years will be with their mentors for a month. for the Two bending groups its more of who would be better to show the ropes for the MHA world if that makes sense.