AHI replacement - forget it. Too much work for no gain.
Libre Office/Open Office - forget it. It is not enough to port LO/OO but you must also maintain port next ten years. And for starters I really recommend considering something easier in smaller scale. And not to mention to get there you might have to port some dependencies first i.e. boring (although the MorphOS team could help you there). But I really recommend starting with something else.
GTK-MUI - I think it took wrong approach. It should have been GDK-MUI project so you could build GTK out of the box with native widgets. It also has lot of dependencies (pango, glib etc) making it quite boring.
Graphical diff utility could be nice.