Want this too. Plus another Marker idea:

When you place makers on a clip in the source viewer it would be great if there were a way to see in the 'Project' window or the clip's associated bin, that the clip contained a marker. Additionally if you placed multiple markers on a clip, there would be a drop down triangle next to the clip - when viewed in its bin - that when clicked revealed all of that clip's applied markers, and double-clicking one took you to the marker location in the source window.

I think Final Cut 7 had this.

Definitely want this - I use markers a LOT for many things - making timestamps for exporting for video playback, notes to self, lyrics in a kinetic typography video... you name it. Using the keyboard is my go-to and the fact that new markers are blank, and then I have to press m again to edit, then type in the name, then tab to the duration - it's repetitive.

An Option or Preferences setting to set the default duration would be fantastic - I could set it to 1s, 2s or 5 frames 10 frames... whatever, and then when the marker is made, it's automatically something I can resize easily.

Another add-on that would be nice that I saw mentioned in another feature request is a keyboard short to have "marker-in" and "marker-out" shortcuts we could assign

Then, as we're playing back the video, we could use the shortcuts we made. Example:
m = create new marker
alt+m = marker-in (start new marker here but don't create it yet)
shift+alt+m = marker-out (finish creating new marker started with marker-in)
shift+m = go to next marker
ctrl+shift+m go to previous marker
ctrl+alt+m = clear selected marker
ctrl+alt-shift+m = clear all markers

Usage note: If marker-in is pressed again while marker-in is already active, then it ends the previous marker and starts a new one. That way if someone is watching a video (at whatever speed), they can just keep pressing marker-in (alt+m in my opinion) and automatically get full marker spreads all over the place.

Why would we want marker-in and marker-out?
- Interviews
- Lyric passages
- Q&A sections

Bonus option:
Let us set a default marker color as well. That way we can go through once with alt+m and set markers for ... green, let's say. Then someone else or same person decides they need to make markers that are red... now alt+m makes markers in that color that can overlap the existing ones.

Yes, Hotkey for changing the Marker Duration so that I do not have to double click.