As I'm building a project and listening I'll hear things I want to fix 'later' while I'm focused on the task at-hand. Things like, 'I don't like that drum fill', 'need a fill here', 'bad note', etc. It would be great if we could insert a comment (like in an MS Office document) on the timeline as a reminder. I know we have Markers but they're more suited for permanent structural matters. This would just be for temporary 'notes'.
 

Actually,
This was something I was thinking about the other day as I was listening to a "work in progress".
We really need some sort of "notepad" that runs alongside the view (whether track, console, staff or whatever view you're in). 
I think it would be a handy feature and would get much mileage my way.
Currently I use the Project info tab to input my notes, but if they were to be visible like Markers, that would be nice.

I currently literally DO have a Notepad window open, or sometimes a Word doc, depending on the complexity of the notes for the given project, open while I am in the project.
 
I will frequently make note entries there OR take quick screen shots in Windows OR sometimes even use my phone's camera for a quick shot of something.  If it is a text entry, then I will jot down the M:B:T info or whatever time view I am using, so that I know where it is I am talking about.
 
This way, I can be as brief or as verbose as I wish, AND, I can quickly email the project notes to others involved in the project, so that everybody knows what's up with either their part of it all, or the whole thing.  Keeping this information OUTSIDE of the project makes it easy to prepare for edits and such completely independently of being in the studio, and has worked well for me for many years.
 
Bob Bone
 

 While not exactly what the OP has asked for, as it's not across the timeline, I use both:
a) the comment space in take lanes (sometimes will create a take lane only for this purpose; and
b) this free VST plugin https://www.meldaproducti...roduct.php?id=MNotepad
 

Geez is there any problem Melda doesn't have a solution for?

There is a notpad in the inspector for each track already
But I like the idea of having one in the timeline also

I would love something like that! Especially with the ability to insert a jpg or png also. I could live with just one easily movable opening window that you could set text and images and scale it as you like. I would like to keep everything with the songproject I'm working with, so when I get back with some old stuff I wouldn't have to worry about where my notes are and so on... It could be cool also with the lyrics, because now I have to open a word document to show me the lyrics when I'm singing it (I use to edit the text as I try it). It would be easier if I could see the text in Sonar and move it where I would want it.
 
To have this "thing" in Sonar itself would be good also because when changing computer, operating system and so on there's always a risc that a third party plug-in won't work anymore or you would have to upgrade and the notes would be lost forever.

I currently use a Microsoft Word Doc for each project, but I would love a drop down notepad built into Sonar with a button to print a time stamp of the now marker into the document, and a jump to button to bring the now marker to that piece of the project (Even if the time stamp was like a hyperlink, that would work). 

I think having expanded note taking capability within Sonar would be great - that being said, I also see benefit in having notes external, such as in Word or Notepad, so I can look at things or search for things from outside of Sonar - particularly if there is some reason the Sonar project cannot open or opens but has any sort of corruption issues (not a highly likely thing though).
 
I just got used to having project notes - my to do lists for things needing tweaking in a given project, and just keep those in the project folder, in a Project Notes sub-folder, as I had mentioned earlier.
 
But yeah - if more notes can be placed within projects that seems like a great idea.
 
Bob Bone
 

Try adding a dummy midi clip (i.e. something that will not interfere with playback).

Now you can put information into that clip, e.g, "fix kick drum here", and position accordingly in the timeline. Then name the track "To Do: drums". You can add more tracks, such as "To Do: guitar", etc.

I colour my dummy midi clips with a garish red so they stand out. When I am about to fix things I make them yellow so that I know what to work on. Then green to remind me to listen to the fix when mixing. Once satisfied I delete the dummy clip.