As I see it's already implemented!?
I found only keyboard shortcuts Ctrl+PageUp/Down.It's extremely useful option, so I suggest to implement it as: menu option, toolbar buttons, mini-toolbar buttons and mouse shortcuts.
If mouse wheel left/right will be implemented, then use it to go to prev./next subfolder by default in browser at least. I have configured it this way right now with the HotkeyP and WOW! it's amazing, I can navigate through the folders with one finger, using just a mouse wheel!! Up/down to browse up/down and left/right to prev./next subfolder!
There are few problems though...
1. In browser focus is lost then I use these shortcuts, so the first image should be focused when using these shortcuts. This would probably solve also another issue...
2. It doesn't work in browser fullscreen - there is no reason why it shouldn't work. When I'm using the same command as in browser in HotkeyP, it looks it works, but just empty screen is shown, that's probably because of mentioned focus issue.
3. It doesn't work in view mode at all, but it's not implemented in this mode, so it would be harder to implement it here too.
4. It's amazingly useful but also very well hidden feature, so please add it to menu, toolbar buttons... and add also keyboard shortcuts so that users will start use the shortcuts, because it's one of the best new(?) XnView features!