in the Flash they have recentley introduced Wally West who in the comics become KID FLASH in the show now right now we have no information on how he will be come kids flash be here are some of my thoughts about how he might become the 5th speedster in the flarrow universe.
Zoom has also been introduced into the flarrow universe this year and he is obvious not playing around like the reverse Flash from last season. Now your probable wondering how zoom could turn Wally West into Kid Flash well say that Zoom decided to go after Berry's loved ones and targets Wally and he almost kills him to but just in time they were able to save his life by injecting him with velocity 9 (the advanced version of velocity 6) which will save his life because he will have the Flashes healing factor but also will become kid flash because velocity 9 will probably leave lasting speed force in him unlike with Jay Garrick.
Harrison wells was recently came back to the Flash in season two a lot meaner then the reverse Flash and more of a dick to everyone. In the mid season finale of the Flash season 2 we find out that he is working with Zoom because he took his daughter and Zoom made him an offer that if he makes the Flash faster so he gets more sped force when he consumes he will get his daughter back and Harry being the person that he is he excepted his offer to betray his friends. Now how does Harry making Barry faster effect wally west, well lets just say Harry's plan to make Barry faster is to set off the particle accelerator again hoping to make Barry faster but Wally is with Barry and he takes in some of Barry's speed force and becomes Kid Flash.
Now this one is kinda out there but it is possibly now lets just say Wally West is chilling in S.T.A.R labs and the particle accelerator goes of when hes in there because there systems where damaged after a attack that happens there because of there bad security and Wally has recently hanging out with Barry and some of the speed forces that comes off of Barry is taken in by Wally so when the particle accelerator goes off and the speed force he taken in from Barry activates in his body and he becomes Kid Flash.
Sorry for my bad typing skills and terrible grammar .