Is this speculation, or do you know something we don't? Why would Thanos eradicate the surviving Asgardians, yet leave Thor in space, and allow Banner to return to Earth (unless Loki somehow assists in this).
I would hope that the ship has some sort of escape pods, one of which takes Hulk to Earth, though I'd like to know where Korg and Heimdall (if not dead) are during IW...
I expect there will be jokes throughout the film, but IW is the big one - it's supposed to be dark and (more) serious in nature. A1 & A2 never (truly) felt like world-ending - even though Ultron had a plan to do just that, it never quite seemed epic, nor was there ever any doubt about our heroes making it out alive. With IW, we've got Thanos, the biggest bad guy out there, and I'd think it safe to say that not all of our heroes will be walking away from this, so the tone needs to be less comical in nature.