Please allow javascript customization and css branding/theming in the new document library experience. A custom cross-site navigation is one example that is used by many customers. Also many customers want their intranet branded in a different color but office ui fabric blue.
JSLink is also used very often. And don't Forget about all the third-party forms solution provider like Nintex and others.
The new experience provides many advantages, but without customizations it is just not usable for larger intranets anymore.

CMIS is currently not available for:
- SharePoint Foundation
- Office365

CMIS is the standard for interoperability... Not implementing it in all SharePoint products demonstrates lack of openness.
Please let all SharePoint users enjoy interoperability, not just a fraction of them.

This limit has always been a bit laughable, and is even more so as we develop more client side applications. In SharePoint 2007 we didn't have this limit and were allowed to make our own mistakes. Now that hardware is so much more powerful, we need this limit removed so that we can build enterprise-class applications.

Office 365 / SharePoint online needs to support opening PDF documents in SharePoint directly to client application (Adobe Acrobat Pro/DC).

As of this request, technical support has stated the only way to open PDF files in Acrobat is to download/save them.

SharePoint On Premise has supported this feature for some time.

Adobe PDF files are as common and Microsoft Office Suite documents and must be editable directly to/from SharePoint Online without the need to down/save edit, and upload which adds user overhead and lowers productivity.

Something I find myself wanting to do pretty frequently is storing an email attachment somewhere into SharePoint or OneDrive for Business. I want to simply drag the attachment from Outlook directly into SharePoint or OneDrive in the browser. Alas, I cannot. I am forced to save the attachment somewhere (my Desktop, most likely) then either drag+dropping it into SharePoint/OneDrive or using the upload function in SharePoint/OneDrive to get it uploaded.

It should be easier than that. Microsoft, please consider adding this functionality to get even more user-friendly integration between your suite of products.

Document Sets in modern document libraries

Please make this feature available for SharePoint Online.

"For SharePoint Online, the Opening or downloading documents, viewing items in lists, or viewing item properties event is not available."

https://support.office.com/en-us/article/Configure-audit-settings-for-a-site-collection-a9920c97-38c0-44f2-8bcb-4cf1e2ae22d2?CTT=1&CorrelationId=da58a595-60b0-4710-a856-e43640f776f1&ui=en-US&rs=en-US&ad=US

There is a strong need to Group By more than two levels of column data. Example, we use SP to track mosquitos for vector control. I want to Group by Week, Trap Site, Trap Type, Species, Condition, etc... The ability to drill down and see totals for each condition or "Group By" would be fantastic. This leads to the next suggestion, Add Calculated columns to Group By Totals.

where you can select individual folders / files

If you have multiple people co-authoring in Excel Online, there are times when you need to open the file in Excel - typically for advanced formatting, etc. If someone has accidentally left the file open in their browser, there is no way to force them out and keeps the file locked. Although not always intentional, it can create a major headache by not being able to simply kick/close out that particular user from editing.

Now we're trying the display list using new experience which released recently.
One important missing function is there is no way to do customization UI by using JavaScript. As you know the classic (current) experiense have JSLink. Hope to provide it on the new experience, too.

SharePoint Online. We really need to set some restrictions of file types that user can upload to a document library.
For example, to deny upload file with extension .bmp and .tiff.

Allow the site collection admins to customize the template for the standard SharePoint alerts. The current email alerts are confusing, provide too much information to the user and are out-of-date style-wise.

Allow the admins to add common fields, i.e. "Title", "Created By", and include an image and hyperlinks. Also allow adding a CSS style sheet.

Somewhat related to this (https://sharepoint.uservoice.com/forums/329214-sites-and-collaboration/suggestions/17274506-leave-files-checked-out-when-required-fields-are-n) problem, is that if the Modern UI is turned on for a document library, it wont enforce mandatory fields, unless "enforce checkout" is turned on.

You can edit document properties, check-out/in and publish quite happily without filling out the required fields.

Turning on "enforce check-out" still allows you to edit properties without completing the mandatory ones, but check-in will complain about missing columns.

When using the "Copy" function in a library, it seems the intention is to create a copy of the document in another library.

But when users want to just create a copy of a document in the same location with a different file name, SharePoint throws an error.

Why can the copy function not simply allow a new copy in the same library with a different filename?

Throttling is a big pain for many customers. If you somehow increase the throttling (no. of items in a list and no. of lookup columns) limit without impacting the performance then it will be beneficial for a larger audience.

I need to be able to check-in or check-out multiple files. I'm not able to do this currently, when selecting multiple documents.