Kayle, Annie, and Sivir are all fun (imo) and I believe are 450 IP each. Annie can blow people up and play multiple roles (mid, top, supp), and I think she is coming back into the meta (supposedly annie top is becoming popular). Kayle has interesting mechanics with her e and can do a good amount of splash damage and is also versatile (mid, top, or jg). Sivir is a pretty good adc atm with excellent waveclear and teamfight, her only weakness is no escape/dash.
Riven is also a fun character and I heard that they are making her animation cancels automatic so skill cap could be lower for her in the future.