All TeX rewrites seem to keep the batch oriented architecture.
What could improve usability and add the possibility of a modern UI for TeX/LaTeX is to have an incremental TeX engine, that only computes the changes in a document instead of everything from zero each time.
Plus an engine that realizes that it has to recompute the document immediately instead of making "pdflatex ..." run 2-3x necessary in order to get what you actually wanted.
What could improve usability and add the possibility of a modern UI for TeX/LaTeX is to have an incremental TeX engine, that only computes the changes in a document instead of everything from zero each time.