In no particular order:

* support reading subs (SRT/SSA) from AVIs

* implement WavPack

* ID3-to-Matroska-tags conversion

* support for TTA files with ID3 tags

* implement WMA (generally A_MS/ACM for unknown audio tracks from AVIs)

* precise durations for TTA blocks

* allow other timecode scale resolutions for audio-only files

* write mkvedit (like add attachments, add chapters, change language tag...)

* translation

* Splitting by chapters

* Concatenating files

* Add a mode to mkvextract that outputs VFR video into AVI + a
   timecode file