I wouldn't feel all too surprised, although the old fandom will crumble, and we can only hope that they make a come back with this newbee. I, for one, would find it dang interesting if this certain replacement is a girl, accompanied by a male companion. Someone who's quite naive and is eager to try and do everything there is to do in the Pokémon world, although falling short a lot of times, and at times giving up. She would eventually gain self-confidence (much like Ash) and respect for herself as she goes through the journey. She would also be prone to straying away from her immediate goals by chasing after fashion and her desires and whatnot.
Her background should be much like Ash's: A completely normal kid with a completely normal mom living in a completely normal town. She'd go on an adventure after insisting for the professor's consent. She should have either Pichu, Riolu or Cleffa as her partner as those close into Pokémon's mascots. She'd try to catch every Pokémon she sees in sight, with very less/no success. She'd obviously get her Pokémon during special happenings in her journey, like a wounded bird or a haughty cat.
//here ends the ramble