I'm basically on board with this whole thing with one caveat.
I think it's worth Marvel trying to limit the amount of significance that Tony Stark has in the Avengers universe (I also hope that Agent Carter's future seasons aren't too Howard-technology heavy) since they've only got like three or four more films with him in it.
The Avengers in the MCU has still yet to feel like the team is important over and above the members of that team
I'm not that worried about this I'm reasonably confident have Marvel have thought alot about the transitional phase. We'll see
Well Tony himself will eventually be replaced (By Pots or Rhodes, still split on this...), but that doesn't mean he must be phased out slowly.
The most sound fan-theory I've heard is that Tony creates Ultron to defend the world now that SHIELD is down, and then his failure to control it leads him to support the MCU equivalent to the Registration Act and into Civil War. Then he will probably lose again, or maybe barely win - but either way, he will lose enough confidence in his own judgement and things to hang up the armour, thereby completing his bafflingly long character arc.
If it's remotely like this, he will go out with a bang, with a lot of impact. Now, yes, he wont be there after that, but people will - I hope - be invested enough in the story to get behind the successors. And sure, he's huge, but Quill is very popular, and so is the Captain. I think the cast has all around really managed to rise to Tony's level (or at least close enough to give him a slap).
I wonder if Ultron will be powered by an Infinity stone? There's six in the collection, and we have only seen three so far.
We've actually seen 4, they confirmed not too long ago that Lokis staff from The Avengers contained an infinity stone (Im assuming its the Mind stone because of the brainwashing stuff). Id bet that one of the last remaining stones will be in Wakanda in Black Panther, no idea when the last stone will appear though. Its entirely possible that Thanos already has one
I'd go so far as to suggest that Thanos has to already have one considering that he's taken up this quest and that he is considered a powerful being in the Universe. If he had truly been coveting them and they were just lying around (say...in an abandoned ruin on a random planet completely unprotected) but he didn't have any, that would call into question his capabilities.
I know they've come out and said that the Mind Stone was in Loki's staff, but why the heck would Thanos give it to him? Does not make sense...
Because Thanos is so confident in his power that he feels he can take it back at any time he deems necessary. I mean he practically let Ronan walk away with the Power gem (albeit not happy about it). He also knows that most organisations who would keep the stones from him dont have the stones to use them themselves.
Also any opportunity for him to show off to Death would be an added bonus
Or Thanos possibly has the Time gem already, making this all irrelevant to him, as he can let everyone else play around, while he has all the time in the universe
I get the need for adaption since they never bothered to adapt Ant Man until this late in the game (he honestly should have been a founding member rather than someone as useless in the grand scheme of things as Hawkeye or Black Widow) but it just makes Tony's character way too bloated. Ultron was pretty much the only think Ant-man had going for him in terms of overall relevance in regards to the Avengers. Now that they're not even using Hank Pym they pretty much neutered the character. At this point Tony is going to be more important than Batman in regards to their respective superhero teams.