Introducing procedures as a replacement of macros has been the most beneficial change from my experience. Even as an experienced user I was very frustrated with subtleties between $param$, $(param)$, <<__param__>>, or $(__param__)$ !!! So, good ridance to macros and its /define and <$macro-call>… I will not miss the hours spent trying to debug you or work around you ! Let it be the first to trial whatever phase out policy you like.
If you can identify other improvements in user experience that simplyify TW then do so without fear.. For backward compatibility goes - anyone who wants say macros could get a plugin that re-introduces them or not upgrade a wiki-file that crucially relies on it … Its more a question of how not if.
There a couple of candidates in my mind that revolve around syntax and hence user experience:
- The widgets vs procedures discussion - that asks that if one was chosen then a simpler syntax of
<<widget>>could be preferable to the mix of<$my.widget>vs<<my.proc>>.. Personally I think you could go further and just say there is a priority order (custom widget then TW widget then HTML) that makes just<widget>a possibility. - I think filter suffixes are over complicated (why both
:and,delimiters) and are redundant now that multiple parameters are supported … and time for the depreciated regex syntax to be purged.
I think you have also raised some good ideas and definately should move on them.. with a couple of suggestions from me regarding these items:
Make sure to expand this beyond the echo chamber of this community - some real UX designers from other sites (maybe other open source communities - or a collab with you-tubers that do website redesigns).. But key is to get ideas from people who are not already “indoctrinated” 
with TW’s current ways.
As long as people can opt in and out - I dont think industry mechansisms would be that intrusive… But I think it is key to know how many users are there that benefit from what features… or for that matter use macros as an example that informs this discussion. Another example is knowing what plugins are out there in the ecosystem so that they can be promoted to new users (another popular discussion topic I’ve seen)
I think this deserves its own thread to get it happening and make it possibly an ongoing activity. I’ll draft something soon.
Cheers: CB