There is a fanfic I'm looking for where Harry is pulled back into the time of the founders by Gryffindor's magic after he is tortured by Voldemort. The founders then train him and he becomes a phoenix animagius and a fire elementalist. He saves the kings life and is knighted. Everyone thinks he is dead. He returns and Dumbledore tries to expel him but can't because he can't change a founder decree. Harry also finds the key to the house of Evans's vault. He dosen't have any siblings. He also makes his own sword. PostGof PreOotP
Twins: A Different Life by Jessiikaa15
Harry is banished to the Dursleys. Merlin steps in and transports Harry back to the time of the founders. Yes this is cliche but really there are only so many ideas but the author knows this and does not spend a lot of time there. Instead we meet a confident, cunning, ambitious founders heir who is determined to teach his birth parents and Dumbledore a lesson.
Also answered over here.