Briefly: T204371: Replace initial colon in (hash-prefixed) parser function invocation with vertical bar proposing allowing a vertical pipe to separate the first argument of a parser function, so that's not necessarily an issue. T204307: Parser Functions should support named parameters proposes allowing parser functions to opt-in to the "standard" named-argument processing such as used in templates, which I think is a better way of solving that particular problem in the long term than teaching VE/Parsoid a whole new set of serialization options.

In general I'd like to have a "parser function syntax" alternative for every extension and magic word, so that we wouldn't "need" the type parameter, we'd just always look up the various constructs in their "parser function form". The one tricky case is extensions, which do already have a parser function form in {{#tag}} but then would need an additional extension hook because {{#tag}} is used for *every* extension. It's a little awkward, because I don't "really" want to have to do dynamic dispatch on both the first argument *and* the parser function name, but I guess doing so would help with {{#invoke}} as well, so maybe I just need to support delegation.

Change 819740 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):

[mediawiki/extensions/TemplateData@master] Specification.md: Update to add parser functions and extension tags

https://gerrit.wikimedia.org/r/819740