A couple of things that came to mind recently:
1. Manager auto update. Manager queries the Modx repository periodically, or on startup, for new versions of installed snippets, plugins and modules, prompts the user to upgrade, with option to back up existing versions. Of course this idea could be expanded to a whole integrated snippet search and install feature. Best as a module I suppose to keep the core thin.
2. Snippet call builder. Allow user to build snippet calls by selecting from a list of options. This could be done by a standardized format in the snippet header that is read and parsed by the builder. For instance:
/*
<--parameters-->
parameter1, description, type (e.g. yes/no, numeric, etc.), options, default
parameter2, ...
<--parameters-->
*/
The builder could perhaps be a popup window that pastes into the calling object (template, chunk, TV). I would think this could be a module, but the standardized snippet header is the main thing.