The biggest missing villain from Flash's Rogues Gallery has been Mirror Master, but that will be remedied shortly as season three gets underway. According to CBR, the CW / WB show has cast Grey Damon as Sam Scudder / Mirror Master. Damon currently co-stars on Aquarius as Brian Shafe and has also appeared on Star-Crossed and Friday Night Lights.
Mirror Master was created by John Broome and Carmine Infantino in The Flash #105 from 1959. The character previously appeared in the original Flash television series from the 90s played by David Cassidy, father of Katie Cassidy / Laurel Lance.