Over time the Flash comics got seriously cluttered with both speedsters and time travellers. I really hope the TV show stays as far away from growing either group as much as possible.
A Reverse Flash was a must-have, and it looks like the writers have melded the backstories of the two from the comics, so I don't expect to see a second one. I wouldn't mind a nod to a previous Flash across the river in Keystone City; too bad Shipp is playing Barry's dad since he'd have been obvious casting for Jay Garrick.
I'm not convinced Barry needs a sidekick, but if they ever bring one in I think it would almost have to be some interpretation of Wally West. He was Iris's nephew in the comics, and could be her cousin on the TV show. Anyone else (Barry's grandkids Bart Allen/Impulse and Jenni Ognats/XS, or his kids Don & Dawn Allen/Tornado Twins) are from the 30th Century and would complicate the series by either making time travel an accessible thing, or requiring some lame explanation for why that person can't/doesn't go back home and why the timestream isn't corrupted by his/her presence.
Other unnecessary speedsters: Max Mercury, Johnny Quick, Jessie Quick, Kapitalist Kouriers, Cobalt Blue (Barry's separated-at-birth twin brother, Malcolm Thawne)...
Unnecessary time travelers: Any of Barry's decendents, any other member of the Thawne family, and Iris's mother.