`ctrl+scroll` is set by the [Scintilla](www.scintilla.org) widget, it would have to be changed there first I think.
Anybody can write a plugin, or if a suitable one exists the feature can be added, plugins are [here](https://github.com/geany/geany-plugins).
Different people have different ideas on what makes a huge difference to their experience. Clearly nobody who currently uses Geany missed the feature enough to implement and contribute it, not even in a plugin.