Sounds like it will be some fun, but I don't know how they can tie all this up so quickly.
I think that the older Dr Venture will play a critical role, when it's revealed he never died because the true secret of the bodyguards is that they are never able to kill and that they always spare the life of the Venture they are guarding. I really think it's leading up to an ending like that which will make the wait for the next season nigh unbearable.
I also hope that Phantom Limb and the Guild make appearances.
I think that the older Dr Venture is going to be the head of the Guild. JV Jr. follows in his father's footsteps and becomes a villain and arches Rusty except he takes it too far and intends to actually kill him. Monarch, not able to allow anyone else to vanquish his arch-foe but him, leaps in front and takes the proverbial bullet for Rusty.
In the end, we see a battle between "good" and "evil" as Team Venture, Dr Impossible, The Monarch, Underbheit, etc. face off against Jonas Venture Jr and Sr, pirates, the Guild, Phantom Limb, etc.