a) Is this a good thing to do? Markdown is technically still text, but some might feel that it shouldn't be part of the gopher item types.
To keep the "Gopher" feeling, please consider restricting Markdown images and inline HTML.
Analogy: The HTML content type is currently used in the Gopherverse. HTML is technically still text, but has a lot of extra features that make it very un-Gopherlike. My client handles that by rendering the HTML content a sandboxed IFRAME element with restrictive Content Security Policy:
<iframe sandbox csp="default-src 'none' style-src 'unsafe-inline'">
No external scripts, images, stylesheets, ...
This is an implementation choice that I feel is consistent with traditional Gopher focus on text. It retains the user's explicit control over outbound requests (e.g. no tracking images, no web counters, etc).
b) If the feeling is that there's no issue having support for markdown, what would be the best character to use as an item type identifier. I see that 'M' is already taken (MIME encoded resource), but lower case 'm' isn't. Obviously if I go ahead with this, the item type would need to be an agreed standard. Does anyone have any views on this?
This draft included an `m` type for: Electronic mail repository (also known as MBOX) (Kim Holviala)
This researcher found `M` used for: Mail file?
It does not appear that `m` is actually in use, so that may have been a typo of `M`.
I'm +1 for `m` for Markdown