What about adding support for underlined text?
Yes, could do. I'm not sure that it's a complete SoC job because I reckon I could have it done in a week (if, that is, I actually had a week to spare). Add about a month for somebody new to Inkscape's codebase and I still don't think it's enough.
Now adding support for underlines (including wavy ones) then putting spell-checking on top of it: that's something to sink your teeth into.
Incidentally, do we want to think about freeing up the Ctrl-U keystroke at some point? I'm undecided on the matter. On the one hand underline is much less frequently used than bold or italic (I'm going by the MS Word usage figures here) but on the other I'm sure that someone is going to get caught out by pushing Ctrl-U and having nothing obvious happen, so continues typing and wonders why no text appears. Anyway, something to think about.
The text flow could also be expanded to be able to write fractions, equations, etc.
I'm not sure that fractions/equations are a core feature of text layout. They seem to me to be more like tables (non-HTML ones, that is): something to be put on top of layout rather than tightly-coupled. This brings up the whole foreignObject can of worms as well, for integrating MathML/HTML neatly.
Richard.