What is the best Cut-Out-The-Middleman, DIY project one could take on?

Hey! Hope yall are doing good.

I'm a junior developer and I'm looking for something along the lines of what I described in the title. Some examples include making your own email server or making your own ISP (too expensive).

I'm looking for something to actually do, with maybe a team of two to three people, and it can take as long as a couple of years to accomplish.

I'm looking for something that's practically useful - for example having your own email server offers various benefits.

As for the know-how - I'm open to learning new things as I am building what I'm building, and spend some if not most of the time learning as I go.

Thanks!