Word, PowerPoint, Excel and several third party applications support MIP labels to flag the sensitivity and confidentiality of a document, apply protection and enforce display of content markings.
Visio diagrams include just as much sensitive information as these other document formats, and users should be able to label them in the same way with a consistent user experience. Visio already supports the older RMS protection capabilities, but labels would offer a much better and more complete user experience.

I would like to see a feature where when you are co-authoring documents that have an excel data source, anyone can refresh the data. At the moment, this is not possible.

Visio is an O365 subscription as is my OneDrive (docs and desktop are backed up and syncing). Please, for the love of God, enable the autosave of drawings to OneDrive. Why wouldn't this be the functionality to match the rest of the office apps already?

Allow a user to set more default options such as connector thickness, color, and fonts. Plus the ability to select all "like" objects for formatting purposes. Formatting seems to be one of the biggest bottlenecks and time wasters.

There should be an electircal element called diode bridge. This element is used a lot in electrical circuts and it's composed of four diodes.

It is possible to underline or strike through, why not overline?
It is possible to do this in Word, but the text can not be copied to Visio except by taking a picture.

The Crime Scene Investigation template for Visio is currently available in the US. It would be great if the template would be available in Japanese

Include on ribbon the boolean operations, i.e. union, intersect, combine etc. It's a nuisance to do manually, prob. since Visio 2010.

I earlier used Visio 2013 and was satisfied except a few problems. Then I bought the latest Professional 2019 and now am highly frustrated. Any symbol I use from the Electrical stencil has a fixed line width. I can change the line width of the connector but not the symbol/component.

Use Visio to make a timeline of historical events, but the time can only start on December 30, 1899

Be able to provide HEADING and TITLE variable in data visualizer wizard.

Please allow the Data Selector to connect to Microsoft Common Data Service.

can you copy a layer from an existing one? selecting multiple items with different layers to then apply an additional layer ends up pushing all of those objects to the common layerset = pain. If you could copy an existing layer objectset then the number of subsequent edits are drastically reduced.

It would be really good if the Layers dialogue box was independent from the drawing i.e. you can flip through layers AND THEN EDIT THE DRAWING with the layers box still open. Locking the diagram whilst the layers box is open is a pain!

Add options for wavy/sine lines, either through line/arc tool or through shapes. Ideally, the line should be customizable both with "height" of the sine and "number" of wave. For the with, it should give the option to specify fixed number or fixed sine width when changing the line length

Tables would be great for adding in lots of configuration data of resources to tidy up a mixed HLD LLD architecture diagram

I create a lot of process/business workflows with butcher paper & sticky notes. It would be great if I could take a picture of what I was working on & upload it into Visio. Then, all I would have to do is check to make sure everything was correct (and edit, if needed).

Make all dialogs resizable. Some dialogs, for example "modelling preferences", don't show all the content on hi-res displays nor showing scrollbars. I'm completely unable to access them unless I set screen scaling to 100% and make things so small I can't read anything... This makes the software useless for now. :/

Users are frequently torn between making diagrams in PowerPoint vs. Visio, because Visio is much more powerful, but end users can't edit them once the image is transferred to PPT. It would be extremely useful to be able to make a diagram in Visio and export it as an editable PPT/PPTX file.