premable: I always work on a lot of projects at the same time. I'm sure it's the same with many people.
The list in the sidebar can get pretty long and one does have to scroll around quite a bit to find things. Also there's projects on hold, or things that need to be archived and removed from SP but which are being kept for some reason.
Possible solution: Something that could make this whole thing much more usable could be having folders to group things into.
So I would for eg. group all the idle projects into a folder, close it and access it only when a project comes back to life.
These folders could work just like the parent "Project" folder, with a little arrow on the side to expand or collapse it.
What do you think?
Thank you very much for your suggestion! If more people need this I am not against this feature (please use the thumbs up to let me know), but I probably won't implement it anytime soon myself, as there is already more to do for myself then I can handle and because this would add another layer of complexity.
My 5 cents about concept.
It's much harder to implement than it looks on the first sight. You need to implement UI to manage groups (CRUD), drag&drop between groups, counters, navigation, database schema etc. Instead of this I'd suggested to implement "favorite (or starred) projects". So, when you want to focus on a group of projects, you just mark them as favorites. Trello has a similar feature:

Given that project list can be long we first need to find a project we want to mark as favorite. The best way to do that would be adding search field into "Manage Projects" page.

Pros: