Repositories support

Maven vs NPM IMO should be out of question as both represent the most
popular stacks where XML stack is used.
Both are needed.
There are abstraction layers already built on top to hide the details if
look into monorepo toolchains like Nx. That is a good sample to follow.

As structured config format includes as JSON as XML, we just need a common
definition which would map into maven XML and npm JSON.
But there are reasons why many config systems accepting the code based
config objects. Like tsconfig can be JSON, JS, TS file. It makes the
toolchain more flexible. Hence to static formats there is a reason to add
the runtime type ones. D.ts with config class definition ition at least.

Should dependency descriptor be served by own or existing package manager?
While IMO it is a time for semver CDN based repo, which I desperately
wanted to implement,  the existing ones are part of app layer where XML
stack is used. Till the time the XML stack would be sufficient ( working on
it), the own package manager would increase the complexity of toolchains
and reduce DX significantly.  Hence better to fit into existing app layer
ones for adoption and DX reasons.

The formal cross-repo types descriptor would be easy to cell. Having libs
tagged as xslt, xpath, etc. on existing repo would just elevate the
visibility. Naming own repo would serve the opposite- hide it from
community.