Expectations have changed a bit over the years. People want to be able to apply more formatting to text, incorporate images, videos, emojis etc. If you look at Slack I think you get some idea about what I'm on about. Especially with respect to embedded media.
You can, of course, do all that with HTML, but just using raw HTML posted directly from clients would result in too much chaos (and security challenges). Perhaps a somewhat constrained HTML that doesn't include styling so that styling is primarily done at the newsgroup level and can be overridden in the client? I'd probably prefer going with something simpler. Like Markdown.
Perhaps something could be done about how you quote parts of other postings. So you have markup for quoting text (that refers to a section of a posting).
Now, these things aren't stricly NNTP specific, but about content. But they would be easier to implement if you had some machinery for handling binary content, quoting content etc. in NNTP.
The idea of a single USENET isn't really viable anymore, so you'd have to think about how you can make something similar which is distributed in nature. Some of this is down to creating better clients that can deal with, for instance, you reading 100 forums spread across 20 servers without a lot of silly management. Another thing that would be interesting is to be able to quote content from or refer to postings residing on other servers than the one you are posting to.
Then there's identity, encryption and all that jazz. The simplest way to solve identity is to just use whatever providers people already use (FAANG). But you could perhaps investigate other ways to solve identity. Especially if you want to support various degrees of anonymity.
(Then there's stuff like content moderation, the ability to modify and version postings etc etc)
I think the most challenging bit is to create good clients.
Here we are on a text only board - how is that changing expectations.
Quoting parts of older posing is one of the strengths of NNTP clients, they all to it in a common way.
A single USENET was one of the great benefits I don't like going to many different places to get information. I would not that good clients could deal with multiple servers and for us techies things like leafnode made multiple servers easy.
Altering things after posting is a nasty thing - how often do people take snapshots of Twitter so when it is deleted then you can show what someone had done nefariously,
