Yo, thanks for trying out MasterPlan - I'm glad to hear that you're loving it so far. These are some interesting suggestions!
I'm wondering about using tab (and presumably shift+tab) to indent or outdent; that sounds useful, but you can also move Tasks in any direction currently using Control+Arrow keys. Do you think you'd use tab enough to justify it as an alternative to control+right?
Modal functionality sounds interesting, but I want to avoid shortcuts that are too complicated (shortcut chords). You're not the first person to suggest adding hotkeys for specific Task Types, though, so this might still be a good idea.
GUI Scaling is something everybody keeps asking for, and I want to implement; it just takes some time to implement because I didn't make MP with GUI scaling in mind, so I have to go back to rewrite some things to make it work. It's definitely on the chopping block, though; same for customized keyboard shortcuts.
I'm interested in what you mean by "separator block" - like a Note Task, but resizeable?