Ok, I see what you're saying. You're proposing to have TODO states and
other tags on the left of the heading title in fixed pitch, and the
heading title in variable pitch. In my current setup the whole heading
is fixed in Agenda and variable in normal buffers. I agree it would be
good to have mixed-pitch headings.

This is doable for the TODO states already using the org-todo-keyword-faces variable, which can contain full font attribute lists, like this:

(org-todo-keyword-faces

'(("TODO" . (:foreground "DarkOrchid1" :inherit fixed-pitch))

("INBOX" . "cyan"))

--Diego