We aren't exactly a long way off from Phase 3, but do we want to see a reboot or a continuation of the series, even if it is without Mr Stark?
The Amazing Spiderman proved that a reboot can work but it didn't really offer anything new to the franchise. Even X-Men, which was meant to be rebooted in the form of First Class, still managed to find itself tying with the original. The Nolan/Goya Batman saga on the other hand doesn't need any more movies as they managed to end it on a perfect note. While it can be argued that Nolan should make another Batman movie because, lets face it, he managed to completely refresh the Dark Knight's cinematic image (and perhaps also set the bar for superhero franchises in general).
The only problem is that while Batman's universe is fairly well contained, the Marvel-verse has been set up as being vast and full of potential. Marvel have already stated that they are hoping to bring to the screen their more outlandish properties (including the Guardians for next year) and so it seems a waste to let it all fall aside and start from scratch.
So what can be done?
While the comics are notorious for rebooting characters and starting alternate timelines, they also deal with the case of passing on the torch to the protégé however I don't think this willwork with where the current characters stand.
Iron Man (arguably Marvels hottest property right now) only works because it's Tony Stark playing him. Get rid of that and you have a rich guy with a snarky sense of humour and awesome suits but you don't have Tony Stark. Sure you can bring on another guy to take the reign but chances are you lose the appeal that Iron Man had to begin with. Unless you can provide a sure-thing, the only option would be to kill Iron Man off once and for all.
The same problem happens with the other Avengers. Captain America can't really pass the torch on because to do so would be to recreate the super-strength serum and thus create the plot-hole "Why not give it to everybody". Hulk likewise wouldn't want to pass his 'curse' over to someone else while Thor is Thor and technically he can't really die.
As I mentioned, you can always replace the actors (which the Hulk would definitely know about) but then the characters themselves lose the personality that we grew accustomed to. I really doubt Thor would have had the same pompous cheese had Chris Hemsworth not donned the hammer and Chris Evans really managed to make the Captain feel human. Unless you can guarantee that the future actors can bring the same amount to the table, then there's no point in even thinking of doing that. (I know they replaced Rhodey for Iron Man's 2 & 3 and I do think Don Cheadle did a better job, but I still notice the transition.)
I go back to my previous concern that the only way forward would be to kill them off and pave the way for other properties to come forward. We already know Ant Man is up first for Phase 3 and there are talks of the lesser known properties being brought forward and this isn't necessarily a bad thing. Remember how popular Iron Man was before the movie?
In any case, all we can do is hope that there is a third Phase to this current generation and that we do get to see the Avengers return in at least one more stand-alone cinematic outing for it.