All signs point to SpaceX and his mission to Mars.
More tunnels would be cool, but I find it hard to believe there's really that much room for improvement over established companies.
I think the hyperloop is even more ludicrous given the technological problems involved in vacuum tubes. But I think Elon realises that which is why all he did is write a memo on it rather than create a company around it.
I feel like the Boring company is what he settled on once it was made clear to him how monumentally stupid hundreds or thousands of miles of near vacuum tube would be in practice. Even if you could actually build a working prototype, it's hyper fragile and will just never be economically viable compared to other solutions.