I want a new feature!
New features can be suggested on Github by filing an issue and adding the label "new feature". You can see a list of features that have already been suggested here: sorted OHRRPGCE issues (or Feature Requests directly on Github). However, as of writing, we need to sort through these requests because the ones migrated from Bugzilla are 6+ years old.
But in practice most people request new features in the OHRRPGCE feature requests/suggestions thread on SlimeSalad, or pester the devs directly.
You can also suggest features directly to the developers on the developer mailing list.
Remember that the developers are all volunteers (even James!) so you can only suggest features, not demand them.
The source code is available under the GPL license, so if you are really interested in a feature that isn't currently being worked on, you can program it yourself, or hire (bribe) someone to program it for you.