Note that our v11 strategy does NOT include 2 popular ideas to improve Composr themeing:
1) Make new official themes, esp Bootstrap theme (#3206)
2) Make amazing WYSIWYG editing of themes (#2970)

Why?

i) We want to fix things that limits THIRD PARTIES making/maintaining themes.
ii) We don't want to make major changes to Composr themeing infrastructure AFTER making new themes, as we'd need to re-do our work.
iii) We feel improving core infrastructure is more important, because it's more of a one-off investment, than the ongoing investment needed to make and maintain a set of themes.
iv) A certain proportion of users will always want to make their own themes. So by improving core architecture we're serving a broader set of users than making new themes would.
v) Whenever we improve core infrastructure to make many complicated interconnected improvements, it tends to get us to a point where we can see the next set of innovations more easily.

So v10 is step 'a', and v12 might add some new official themes (or we might draw in some third party themes into the Setup Wizard).

As for "amazing WYSIWYG editing of themes", that's going to require an enormous effort, so it's going to require someone's full-time passion, or a large investment.