I think there's three things that need (or, I want) to happen to make Linux mainstream:
1) Windows games need to be playable on Linux without making more work for developers (this is already happening thanks to Valve Proton
2) The Adobe suite needs to run on Linux without needing extra setup. I have heard Wendell from Level1Techs say that Linux and macOS share enough heritage that it's conceivable one day that all macOS software will run on Linux.
3) Linux needs to start shipping by default (or as a configurable option) on more consumer laptops.
There is another exciting thing called Flatpak
which should make installing complex apps much easier on Linux. Flatpak is already supported on Linux Mint 19.
I can't wait for Linux to go mainstream.