In my headcannon, she’s the first to go to Tarzan’s universe where Jane saves her Tarzan style before healing her; hence the dawn of their bond similar to the one between Twylight and Celestia. There she meets characters from Tarzan, Ice Age (moved to Pellucidar), The Lion King, Brother Bear (moved to Pellucidar), Pocahontas (time travelling visitors from the 1600s), Mulan (time travelling visitors from the 600s), The Jungle Book (visiting), Dumbo (time travelling visitors from the 1920s), Dinosaur (living In Pellucidar), and more. And she earn their bond with her virtues well. She also learns from them animal languages, biology including palaeontology and ecology, spirituality including the Ancestral Spirits, philosophy including from Sun Tzu and Shakespeare, and more. And Jane forges outfits more her than impractical dresses; safari jackets, waistcoats, how ties, paisley scarves, shirts, tartan kilts, and Panama caps.
In my headcannon, she is a descendant from an alicons who invented the Sonic Rainboom… An alicons who was the first to dimension travel. Hence one reason why the title character of Doctor Who in his War form (John Hurt) recruited her to a mission involving a Silurian Ark with ‘father’s fauna from Earth; to use a map only working for her bloodline. And it was zero the only reason…. He recruited her for her heart, and smirks on her defending many from Daleks but depressing temptations from Rassilon, Davros, a lead g on her to Pirate Sea Demons, and the War Master (Derek Jacobi). And she also saves an Ice Warrior princess this becoming a spiritual sibling to the Martians. And she’s the first to meet Sarah Jane Smith and Ace as well as torch Cybermen, Sontarans, and the Rani. And also to befriend Draconian s and Thals. And to meet Maximals, Predacons, Dinobots, and Omega Supreme. And the first I meet Marvel and DC ones, as well as Star Trek and Star Wars ones.
Also in my headcannon, Rainbow is reveal to be a Messiah as her ancestor.
Also in my headcannon, many find out about the wrongs done to Rainbow; the Marer Do Well and Walking Dead hoaxes, the division with Rarity involving the swamp, the cheerleading gambit, Twylight arranging the idol lesson to use her embarrassing parents and Scootaloo against her as well as arranging the Ghastly Gorge avalanche and framing her for many pranks to play many on her, Applejack using her to control elders, Twylight arranging traps for her in her competition with Applejack, Luna zapping her in her holiday, Pinke’s pie torture, Rarity hurting Rainbow with her rubies, Twylight mind controlling Daring Do to abandon her, and more. And many seem to punish them, including some whose family members were affected by the wrongs including deaths and PTSD. And Rainbow proves loyal such as defending them from more aggressive ones. And Fluttershy was easily pardoned, for she confessed the truth and redeemed herself; while the others must earn trust. And many see alicons as deceptive monsters cos of Twylight and Celestia puppeteering events.
Also in my headcannon, Scootaloo runs away after the truth of Twylight’s puppeteering was revealed. And Rainbow saves her from poachers in Tarzan’s universe and helps her heal. And acts as one of the guides for the CMC and more in Tarzan’s universe.
Also on my headcannon, the pony Rainbow befriends the human one from Equestria Girls who becomes a sibling to her. And many parallel versions; one who’s an Edwardian explorer and beloved to a jungle Soarin in a prehistoric world on Africa, one who’s a buccaneer version of Robin Hood, one who’s a she-leader among Ming soldiers, one who’s a Wild Western heroine, one who’s a Macduff to a Twylight Macbeth, one who’s a samurai, one who’s a World War heroine, and one who’s a queen.
Also in my headcannon, the Rainbows get scars especially some iconic ones on their chests protecting their Scootaloos. And their proudest scars since they got them saving sisters.
Also in my headcannon, the Firefly from Gen 1 was thawed from suspended animation And be Rainbow’s grandma as well as mother to Windy.
Also in my headcannon, Daring Do be Windy’s sister and Rainbow’s aunt. And instead of anger, Rainbow is more forgiving.
And also in my h, the rest of the Mane 6 and more become alicons. Rainbow being one in the sky; chief of staff for the Wonderbolts and willing to lead from the frontlines. She and Soarin marry and get babies. And Rainbow proves grand as a mom; more concerned about health than gender, calming each baby with her heartbeat, teaching the kids martial arts, and protecting them. And they inherit her virtues.
Also in my headcannon, Rainbow got many frozen and was the first of the Mane 6 along with Soarin and their kids to awaken in Gen5. And she acts as a mentor toxins including Zipp who is founding the new Wonderbolts.

Yes she is the bravest and most loyal. And nicest to kids; zero verbal abuse as Twylight, Applejack or Rarity.