I don't think the should replace Ash and Pikachu. But I think Ash should've aged in each series. Kanto series (age 10) Johto series (age 12) Hoenn series (age 14) Sinnoh (age 16) Unova (age 18). then maybe after the unova region go in about 20 years in the future. Have Ash have a son give him a pikachu the son of Ash's Pikachu and have a journey just like his father.