Buying Champion Troubles

I am a mid lane and ADC main, but I am trying to get coverage for each of the roles as I get closer to playing ranked games. I have 2000 ip right now and am deciding who to save up for. I have close to 10 mid laners and I only play a couple of them. I have Garen and Gnar for top but Garen is really bland to play and Gnar isn't really suited to my play style. I have enough ADCs that I am okay with, and the same with support. I have a couple Junglers that I am okay with. I am thinking about buying Fizz because of the fact that he can go mid, top, and jungle. However, I want to get Riven to just snowball top along with her having no ability costs. Any advice?