You dove in head first with the whole "there's nowhere to go after Thanos" crusade in a way that clearly suggested that you thought the whole "villain escalation" thing was a must; that if they did it any other way it would weaken the movie. There's zero evidence to support that. It could be good with villain build up. It could be good without it. There's no one way to go or no one way that's inherently better than another. Therefore, it doesn't really matter whether they do it or not. All that matters is that, whatever they do, they do it well. Which means that if they want to follow Thanos with Ultron, Graviton, or the Wrecking Crew, they can.