Every time I work with a client or teach a workshop, people want more ways to visualize their project portfolios. Here are some ideas:
Here is a kanban view of the project portfolio with a backlog:
And a kanban view of the project portfolio with an “Unstaffed Work” line, so it's clear:
If you haven't read Visualizing All the Work in Your Project Portfolio, you should. It has some other options, too.
I have yet more options in Manage Your Project Portfolio: Increase Your Capacity and Finish More Projects.