Before I get to the mods, I need to say that this PC does have ASUS parts: Pro WS X570-ACE motherboard, TUF Gaming K7 keyboard.
My PC mods are a lot different from everyone else's. There is no flash, no bling, and at the moment, no RGB. Instead, I'm going to show you how an engineer does it, or more precisely, how an engineer over-does it!
PC Mod #1: The Air FilterThe current trend is to use large radiators, slow moving 120/140 mm fans, and very minimal air filtration. Over time the PC will accumulate dust, hair, and nasty crud that never comes off fan blades. But with this mod, my PC will stay squeaky clean. I've built models for 70, 80, 92, and 120 mm fans. The whole filter consists of four printed parts, several bolts, one lawnmower air filter, and one fan. While this mod doesn't feature any RGB lighting, you have access to the OpenSCAD files on GitHub, so you could remix it to include RGB light strips behind the filter, shining through.
Inside the case:
Outside the case (without front panel):
PC Mod #2: Redundant Power SuppliesI designed and built this mod just to show that it could be done, but the result is so freaking awesome that I had to share. These are dual 1280 Watt, hot-swappable, 80 Plus Platinum power supplies. The Power Distribution Board features three GPU power cables, each with two 8-pin connectors. I think this server-grade setup meshes well with the ASUS workstation-grade motherboard. NOTE: Everywhere you see the color orange, that is this mod. It's huge!
Backside:
Underside:
There is no reason why you can't build these mods, too. All the knowledge you need is right here.