Oh, now someone will say, but what about photos and images? What about "clippings" from the web? Images are to be in a properly tagged image library. And a dozen software solutions have already solved that. Documents are PDFs or TIFFs or JPGs or.... something like that. Put them in, gasp, a "folder" ("directory" for geeks) and call it something like "documents".
I don't know what "notes" people are so desperate to save, but if they are your own ramblings... plain-text. If they are writings, make a folder called "writings". Use markdown or ReStructured Text if you must, but trust me, the "must" is your must. No one else cares. It will all burn eventually.
Plain text will still be here as it has been for four decades. Your app, OneNote, EverNote, NoteyNote, and all the rest will all burn. Or be hacked. Or stolen. Or not be backwards-compatible with whatever new NiftyNote comes along.
Plain-text. Stop making these. The problem was solved 40 years ago. Solve something else. Maybe Cancer. Maybe how to install Windows 10 in 3 minutes or less. Maybe public transportation.
But for all our sakes, no more Notes apps. You are actually accomplishing the opposite of what you set out to achieve - further distracting people from an ancient standard which works Just Fine For Almost Everyone.
By the way, if you are That Guy (you the reader) who thinks your special (said with a lisp) notes deserve better than Plain Text, you probably are wrong.
Same with To-Do list apps. Plain-text. Just write down your tasks - and then go do them. Stop prioritizing, colorizing, making 'sub-tasks', attaching deadlines, putting time estimates, "sharing" them with others (oh for the love of all things holy, stop sharing anything), and adding javascrap drag and drop junk to it. Either buy Microsoft Project and use that or use Plain Text. There is no in-between - just clutter. Lots and lots of clutter.