and was wondering how hard it'd be to create a USB hub that supports
the USB Alt mode for things like video.
This is not about how dense a Type-C connector is, or how many layers of PCB one needs to use to make compact differential traces. Or how dense the hub controller pinout is (it is usually QFN, not BGA). And the question is not even how hard it is to design a silicon chip that will incorporate logic and datapath for at least three hubs (USB2, USB3, and DP), it usually takes several dozens of men-years of seasoned engineering teams (an this is no fun). The question is how soon it will be economically beneficial to have this kind of device on your desk.
USB hubs a relatively cheap because the industry is producing them (and people use them) in quantities of maybe 100,000,000 units per year.
As a matter of fact, the device you want to make for a fun is called USB4.
If you can justify a business plan to make this kind of IC and can afford to spend $10-$20M plus years for PC host controllers to catch-up, and software stack to become functional within Microsoft windows, and have full-featured 10-GHz low loss cables at your disposal, then you can have all the fun you want.