TYPO3 Core

How about:

page.cachetags.<whatyoulike> = myTag

We should then also allow stdWrap on those entries so various values can be reused to create a tag.

just make sure to use camelCase ... cacheTags :)

I would prefer a tsConfig way to add tags (or even in page options).

I often need dynamic values for cache tags, not just static strings.
Imagine rendering news or addresses with plain TypoScript CONTENT/RECORDS. When the page is tagged with tt_address_[uid], it was pretty easy to just install enetcache and let its hooks drop the cache for the pages whenever a record was edited through TCEmain. So in my opinion, adding tag entries should be available in stdWrap so you can use it in renderObj, PAGE etc.