Application Manager: Maybe with the help of some extensible (but sane and standalone, with titlebar) file manager, but question remains how to send enumerated apps from such a file manager view to Front Panel subpanels or as submenu. Probably external drag and drop applet which can be swallowed in subpanels to accept drop with middle mouse move and edit subpanel configuration? This will than replace Subpanels Manager app, but it must also have functions for editing, deleting etc ...

Action builder (dtaction) - not likely ever. Use FvwmScript or maybe some Python gui bindings.

Session Management (dtsession) - NsCDE can use the external custom Session Managers from various DE's. See examples in share/doc/examples. Since there are similar programs in existence, plus FVWM's own functions for automatic start of programs, NsCDE is more or less covered here.