TYPO3 Core

Actually the core defines hardcoded which target file-extensions are mapped, if nothing is defined via configuration-array.

In the function "determineTargetFileExtension" in file "sysext/core/Classes/Resource/Processing/AbstractGraphicalTask.php" it is defined that:

1) jpg, jpeg, png and gif will stay the same
2) uncropped svg will stay svg
3) unprocessable files with disabled option $GLOBALS['TYPO3_CONF_VARS']['GFX']['thumbnails_png'] will be gif
4) all other files will be converted to png

This disables the possibility to render all or specific image types to a default type. This is usally needed, if the use of the new file types heic/heif or webp is needed (suggested for better performance, Google!)
The usage of heic/heif files (Default on Apple Devices) oder webp is still configurable, but must be implemented by an integrator to every element. This could be a fluid template with the image viewhelper and the attribute fileExtension.

New configuration options in $GLOBALS['TYPO3_CONF_VARS']['GFX'] would make it possible to set the target file type. Optimum would be the possibility to add a mapping for each configured type.
A default example list could be something like that:

jpg -> webp
jpeg -> webp
webp -> webp
heic -> webp
png -> png
gif -> gif
svg -> svg

Maybe a default file extension option if no mapping exists.

There are a couple of other configurations we should have an eye on.
The options
$GLOBALS['TYPO3_CONF_VARS'][GFX][gif_compress]
$GLOBALS['TYPO3_CONF_VARS'][GFX][jpg_quality]
for example should maybe extended by new features for webp optimiazation

Either the "Image Processing" module in the backend could be extended with heic/heif and webp file format.

Last point. We could rethink to get better performance using the modern image formats in the backend.