Personally, I like MPC-HC's implementation of key combinations, though I've redefined a couple of them to suit my purposes, such that the following is true:
Advance 250 ms: Alt + Right arrow
Advance 1000 ms: Right arrow
Advance 5000 ms: Shift + Ctrl + Right arrow
Advance 1 frame: Ctrl + Right arrow
Advance to next key frame: Shift + Right arrowAll of these combinations are true for their respective opposites, albeit using the left arrow key instead. If possible, I would highly recommend (something like) this implementation for the sake of simplicity and usability.
Key frame functionality is bolded because, along with regular frame seeking, it is the most imperative feature since most video editors and muxers with cutting/splitting functionality will simply cut at the nearest key frame, regardless of the specified timecode or frame number. The only exceptions to this are applications like VideoReDo, which can transcode individual frames when needed, but cost a fairly hefty sum. I suppose that's not really JRiver's concern, but it would be nice to have, and assuming the development overhead isn't egregious, it might as well be implemented if any of this is going to happen anyway.
It would also be convenient to give the user the choice on whether to display timecodes or frame numbers.
It would be
super convenient were it possible to be able to copy and paste them in display view.
For reference, MPC-HC accomplishes this via key combination--Ctrl + G--which pulls up a small window that displays both in editable fields. See attachment.
On a semi-related note, is there a key combination for chapter-by-chapter seeking? I imagine there is; I'm just unaware of which it is.