RTS is probably the easiest to get into if you don't play them. It's a shame that it's pretty dead as an esports genre now. Watching SC1&2 during their peaks was wonderful.
Fighting games are probably the 2nd easiest thing to watch imo, but you've already mentioned you're not satisfied with that.
So, tbh, I don't think there's any great options left at the moment. Trackmania would have been a suggestion, but its main esports league went belly-up this year, so that's now gone too.