Is there a possibility of adding custom props. to Instance Groups in the future?

Unfortunately, they will need to make instance groups into actual groups first, which I have requested many times.  As it stands they are basically limited tags where you can only have a single tag for any instance and then you can apply thresholds to the tag, but those don't apply to new items tagged the same way.  This is why they throw up the warning about how new instances won't get thresholds automatically.  We've had to workaround that with API scripts to refresh those instance group tags and thresholds when new volumes are added to servers (as just one example). I assume if they ever are made into actual groups, properties would be part of that.