This is already possible using a Rendering Transformation, see https://rendering.xwiki.org/xwiki/bin/view/Main/Extending/#HAddinganewTransformation
Note: We use that for emoticons for example, see https://extensions.xwiki.org/xwiki/bin/view/Extension/Icon%20Transformation (and xwiki-rendering/xwiki-rendering-transformations/xwiki-rendering-transformation-icon/src/main/java/org/xwiki/rendering/internal/transformation/icon/IconTransformation.java at master · xwiki/xwiki-rendering · GitHub for the code).
ATM I don’t think we should have this by default as it’s not a common need. Could be a contrib extension though.