As a Plugin developer, we would very much like to design an icon for Activities matching the style of Moodle 4.0. But since a new icon would look very out of place in older moodle versions, we would suddenly need to sustain two plugin versions for each activity plugin, which isn't worth the effort.

My idea is for Moodle 4.0+ to use the (for example) "modname/icon4" pix if it exists, and the normal "modname/icon" pix otherwise. That way, if there is no "icon4" pix, everything works as it was before and the "icon" is used everywhere, but if there is an "icon4" pix, the "icon4" is used in Moodle 4.0+, and the normal "icon" is used in Moodle <= 3.11.