Depends on what you like: I've put 300+ hours in Rainbow Six: Siege over the last two years, and it is a fantastic choice if you enjoy extremely intense, competitive shooters. Cities Skylines is another fantastic choice if you like city builders. Dolphin Emulator will now probably be able to run GameCube games at full speed at 1080/1440 enhanced resolutions.
I love Cities Skylines, even if most of the DLC seems to suck (after the disappointment that was Snowfall, I haven't bought any). R6 Siege always looked really interesting to me. How is it for a filthy casual? I hadn't even thought about emulation! That's definitely something I'll have to look at, since the new machine is pretty dang beefy.
Siege has a steep learning curve, but the destruction and strategy involved sets it apart from anything else out there. It take practice and time to do anything cool, but it is so satisfying to pull off a victory you shouldn't have. Watch a site called isthereanydeal for a sale of you're unsure, don't buy the Starter Edition any more. For Dolphin, PCSX2, and I think RPCS3 (experimental PS3 emulator) processor power matters more than graphics card, but a current i5 should run the first two well enough.
PCSX2 especially I'll have to look at. My new box has an 8700K, so I should be good :Dcurrent i5