I’m about to build a new site based on a theme close to what I’m looking for. I understand the concepts around child theming & have read the caveats about overriding php or css files like the one described here.

Using such workarounds and calls from functions.php, is it possible to build a “Frankenchild theme” by taking chunks of a second (would-be parent) theme and patching them into the child theme? I’m not quite asking about creating a child of two parents; the child would only have one “official” parent.

As an example: Start with a Gantry basic theme and add custom taxonomy search capabilities & search results formatting (say a card-based carousel). Is that stepping over the line into plugin development?