Interesting. I think that all of those requests could actually be handled within TW5 without breaking backwards compatibility (in particular it wouldn’t be much work for anyone interested to explore building their own custom core).
The sorts of things I’m thinking of for a subsequent version that didn’t retain backwards compatibility are at a more fundamental level: rethinking the structure of a tiddler, and how variables and the rendering process works. For me, the trouble is that TW5 evolved organically: when I started it I wasn’t even sure that I could achieve what I was setting out to do. Thus, there are lots of quite prominent features that were introduced early on and that I would now handle differently (eg, the fact there is a difference between transclusion and the view widget, or the presence of text substitution macros which I now think was a mistake).
Best wishes
Jeremy.