(Structuring task descriptions into 1) Steps to reproduce, 2) Actual outcome, 3) Expected outcome is welcome.)

In regard to reproducing the concern in it's simplest form is there a test wiki I can put a minimal template example on?

https://test.wikipedia.org/

Okay I added a minimal example here -
https://en.wikisource.org/wiki/User:ShakespeareFan00/foo/bar
https://en.wikisource.org/wiki/User:ShakespeareFan00/foo/testcases

I also for comparison add some test cases for
https://en.wikisource.org/wiki/Template:Sanenull
User:ShakespeareFan00/sanenull-x

These could be implemented internally rather than being templates. I'm not sure pass-thru for certain templates could be implemented though, so will need more time to formulate something....

In regard to the comments about a {{pass:parameter}} Let me provide an example

Template markup for {{foo}}:

{{bar|required|{{pass:optional}}}}

When subsequently callled:

{{foo|required|optional=1}}
{{foo|required}}

What should be called in each instance should be :

{{bar|required|optional=1}}
{{bar|required}}

Currently I don't think the parser can make such a substitution automatically..

More than willing to file this as a seperate ehancement request if desirable

In regard to the comments about a {{pass:parameter}} Let me provide an example

Template markup for {{foo}}:

{{bar|required|{{pass:optional}}

When subsequently callled:

{{foo|required|optional=1}}
{{foo|required}}

What should be called in each instance should be :

{{bar|required|optional=1}}
{{bar|required}}

Currently I don't think the parser can make such a substitution automatically..

And further a real world example.
{{#switch: {{NAMESPACE}}<!--
-->|Page={{float left|{{{1|0}}}|{{{2|0}}}|{{{3|0}}}|{{{4|0}}}|{{{style|}}}|{{{id|}}}}}<!--
-->|#default={{float right|{{{1|0}}}|{{{2|0}}}|{{{3|0}}}|{{{4|0}}}|{{{style|}}}|{{{id|}}}}}<!--
-->}}<!--

should be instead

{{#switch: {{NAMESPACE}}<!--
-->|Page={{float left|{{{pass:}}}|{{{pass:2}}}|{{{pass:3}}}|{{{pass:4}}}|{{{style|}}}|{{{id|}}}}}<!--
-->|#default={{float right|{{{pass:1}}}|{{{pass:2}}}|{{{pass:3}}}|{{{pass:4}}}|{{{style|}}}|{{{id|}}}}}<!--
-->}}<!--

This vaguely reminds me of a loose end in the heredoc syntax, described in T114432#4717140 -- which is to say, we don't have a good way to flag expansions which are supposed to be an "argument list" vs "literal text". Instead of:

{{templateName|optional ={{{#pass:optional}}} }}

I think what we want is a way to generate a list of arguments, including optional=<some value>, and then to pass that list to templateName.

{{templateName|{{#filter-arglist|optional|{{#arglist}}}}}}

where {{#arglist}} generates the full list of arguments to this function, as a typed/structured object and the {{#filter-arglist}} built-in will extract only the requested ones. When passing an value of this type to a function is gets expanded like a splat so the called template sees the full list as separate arguments.

This is just a strawman, feel free to poke holes. But I agree there's a missing corner in parameter passing here we should try to fill in.

Note that in the common case we just want to pass our argument list down to a subtemplate unmodified, for example to refactor some common processing. This is pretty awkward currently (ie https://en.wikisource.org/w/index.php?title=Template:Lang/new&action=edit ) but could be as simple as:

{{subtemplate|{{#arglist}}}}

in the proposed strawman syntax.

See also T162375, which was a request for an explict way to indicate null values..

https://en.wikisource.org/wiki/User:ShakespeareFan00/foo/testcases My set of test cases for some experimental template code I wrote.