Including additional Apache modules is a great idea and seems like it could be very helpful in the core charm itself. What I'd recommend doing is implementing this via the
hooks/config-changed so you could not only change what additional modules were installed but also add/remove modules on the fly. Then I would open a merge request against the current charm to have it included by default. Charms are pretty extensible and often time if you find a general need a charm doesn't fullfil, you won't be the only one.
So if you do make a change to a charm (and don't wish to maintain a fork) consider submitting it to be merged in to the Charm Store version of the charm by uploading the branch to
lp:~<your-launchpad-username>/charms/<series>/<charm>/<branch_name>. Most of the time series will be the LTS name (precise, for example), charm would be "apache2", and the branch name can be anything, most people choose "trunk". Someone from the ~charmers team will review and provide feedback on your changes and, if it passes inspection, will just be merged to be part of the actual charm itself!