TYPO3 Core

imho this is view logic and not business logic and therefore doesn't belong into yet another annotation that unnecessarily slows down Extbase again.

There is a reason why frontend frameworks like angularjs implement a huge set of filters:
https://docs.angularjs.org/api/ng/filter/filter

If you need to adjust your model data, why not simply use the getter?

We already have validators in Extbase. They can however just check data and allow or disallow them. And from the action there is imho no easy way to hook in before the validators. For example if you have a validator that a certain value must not be empty, there currently is no (easy) way to first trim that field before checking.

The "workaround" to still be able to use those validators is to use a separate initialize-method for each action, where you can access and manipulate the arguments. Or you'd have to skip using the validators by annotations altogether, inside your action process the input first and then do the validation yourself.

Imho having filters as a counterpart for the validators makes sense.

About implementing that as an extension: As you see in the extension from AOE imho the work required to implement such a concept using an extension is quite "huge". Just adding a filter-interface and calling filters if they are defined would only be a small addition if we'd add it to Extbase/Flow.