You can already arbitrarily split panes and navigate them with the Origami package (which works with ST3 also):

https://github.com/SublimeText/Origami

I would like to see something like this as well. Something like this is necessary to provide proper grammar-based syntax highlighting for languages which allow for customized notations. Two examples are Agda and Coq, but there are others. Languages which allow for the definitions of macros or which have good support for embedded domain-specific-languages would also benefit.

This is weird. I don't know much about how such font support is limited but it certainly sounds like a bug or oversight. There are plenty of use cases for proportional fonts in Sublime Text so I'd hope this gets fixed eventually.

It would be interesting to have support for something like this built into Sublime Text. Failing that, there are ways you can approximate such behavior. For example, on OS X you could write a plugin interfacing with Growl for the overlay feature.

As someone who works in the area of static analysis tools for programming languages, I can say that

this is something impractical to implement in the core of Sublime Tex, for all sorts of reasons. For starters, each language has different semantics and type systems which would require a near complete re-implementation for each language supported.

If at all, this should be done in plugins specific to certain languages. It turns out that this is already done in some cases. SublimeClang shows how this can be done for the C family of languages, by using the static analyzer from llvm/clang. Other languages would require different backends but in some cases the tools needed to provide the analysis information may already be available. As pointed out by Sven, there are some other plugins that do similar things, such as SublimeLinter and others. In particular there are quite a few Python oriented plugins that do some sort of analysis.