I'd prefer a comic book probably over a movie. 'All That and Gargantua 2' felt fairly close to what a Venture Bros movie would likely feel like, but no way they could sanely wrap up the series without just going balls to the wall with probably only exploring 2 story arcs in the flick. I would love it, but would be hard IMO.
Cartoon Network almost more than likely owns full rights to the IP, so it would be expensive to do some closure that does not involve them and they seem not interested. IIRC Metalocalypse did something after cancellation, but they had to change character names, show name and other stuff to get around CN/AS owning the IP to the original show. Venture Bros couldn't really do that with just how many characters they have unless Doc, Jackson and some other publisher put down a shitload of money for a finale.
It came out of nowhere, it should end the same way.
Unless they could make it a loop. That'd be hilarious.
I hope they do at least something to wrap it up, doesn't have to be full closure but something.