This seems like a duplicate of T192385: Support for conditional parameters in TemplateData, and others. Such tasks are technically feasible, but there is currently no active development taking place on TemplateData.

Part of the description here is also a more generalised version of T95695: When I add "last1" to a citation template, automatically add the related "first1" parameter.

At least part of this seems doable, i.e. have something like this:

		"archive-url": {
			"label": "Archive URL",
			"description": "The URL of an archived copy of a web page, if or in case the URL becomes unavailable; requires 'archivedate'",
			"type": "line",
			"aliases": [
				"archiveurl"
			],
			"requires": [
				"archive-date"
			],
		},

The "requires" could be used for the dependency case, where using one parameter requires the use of another parameter.

But for the use case mentioned in the thread on en wiki, I'm not quite sure how that would be modelled. Specific examples might help conceptualise that.

I'm also interested in adding support for conditional required parameters. For example, Ukrainian Wikipedia has template «Шаблон:Ніхонґо» (https://uk.wikipedia.org/wiki/%D0%A8%D0%B0%D0%B1%D0%BB%D0%BE%D0%BD:%D0%9D%D1%96%D1%85%D0%BE%D0%BD%D2%91%D0%BE), that has one required parameter, that either second positional parameter (|2=) or third positional parameter (|3=) or parameter hepburn (|hepburn=). If one of them is presented, then the template will work, but if none of them is presented, then it doesn't and throws an error. So, I need some features that will indicate to the user, that they need to enter one of the listed parameters, but not all of them.

I'm afraid what people are asking for here is called "programming" and requires an actual programming language. Which we have. Lua. The pretty much only thing that's missing is a good way to display such error messages as part of the VisualEditor experience. One possible solution is when each template comes with a separate tiny "parameter validator" Lua module that can be called individually by VisualEditor.