Well... yes but we are used to do lots of funny stuff with themes. ;-)

It would be nice to keep the dubious but useful practices of theme tweaking while taking advantage of the new stuff. 

well... yeah. The funny stuff usually requires a developer and a redeployment to publish any updates, thus it makes it quite hard to push some updated requirement to production. The new technique just requires UI updates, no redeployment.

I'm not saying you should change your plans, but maybe it's time to unlearn old habits. (Keyword, seriously, is "maybe". I'm not implying anything). I'm famous for asking "what do you actually want to achieve with this?" - maybe there's a quick way to point out how to solve funny things with MP/SB/Classic. If there isn't, you can still go back to the custom theme.

So far, let's make sure this isn't a x-y problem.

No precise requirement ATM.

Let's imagine we have a heavy customized theme with lots of legacy extras. We would like to make it compatible with stylebooks withou having to change a lot of logic. 

To evaluate this posibility, we would like to understand a little better how to support the new features in themes. 

Hence the question. :-)