Happy to correct you: datatypes, assigns, user interface.
doesnt sound like much.
biggest problems seems the linux-oid file structure as you yourself notice.
certainly if there is no valid replacement for assign, such a program could be done after the previous problem has been solved.
with the interface you likely mean workbench. well, actually all contemporary guis share similar philosophy introduced by xerox, mac and amiga. they are bloated, as amiga would become if it survived, but bloated with additional and sometimes unnecessary functionality. actually aros wanderer strives to catch up with any of them as well as the genuine workbench, and this demonstrates the woes of reimplementation pretty well i guess.
and datatypes.. isnt there a similar system in any of the contemporary oses? libreries to open certain file formats? im not sure but i would expect that.. in fact the genuine amiga systems had another annoying flaw to open files with application stored in tooltypes, which could not be changed once for all if the host system was lacking the particular app, but had an alternative named or located elsewhere.