There are already means to extend the time for WW. It would be odd to suggest having more ult than is needed to use the ult give further bonus since that is not the case with any other ultimate. Granted there is some uniqueness to the WW.
Those that want change, this is not what they are clamoring for.
Overload.
I am not against the ability functioning like this, I have suggested and broke it down multiple times. One of the biggest pitfalls is how it interacts DK and Battle roar. Does it grant nothing (not good) and a nerf to dk Werewolves, Does it grant the full amount and become the next exploit.
But with your idea the passives that extend WW form from taking damage and feeding would have to be changed to ultimate gain. That is not permitted under the current design and should not.
Regardless, it is trying to fix something that really is not broken. Enjoy the game, have a good evening. There is nothing more I can add to this topic.