Time travel is a much loved trope of all science fiction and fantasy, from the recently departed Fringe, to the Terminator franchise. When it stands as the foundation for the series, it can be a rousing success; you only have to look to the success and longevity of Doctor Who.
Yet it can frequently suffer the criticism of becoming a Deus Ex Machina, the ability to resolve a story or alter dangling plotlines to a unconvincing and unsatisfying conclusion at best, or sweep things under the rug at worst. It can be applied as if it were a massive wrecking ball (a la Age of Apocalypse or the Flashpoint), or as a finely-tuned scalpel; for an example of this I would point you to the most excellent Avengers Forever by Kurt Busiek, or controversially, I would say the Five Years Later era of the Legion Of Super-Heroes by Keith Giffen. Opinions vary on the latter, you either love it or hate it, I personally fall in the former camp.
Time travel without a doubt has its home in comics. The X-Men would not be who they are today without the alternate futures of the Days Of Future-Past which gave us Rachel Summers, or the dystopian Thirtieth Century of Cable and Apocalypse. You might think that other teams are immune, but where would the legend of Franklin Richards be without the incarnations of the Psi-Lord, the adult Franklin, or the Old Man, and let’s not even get started on Valeria.
Of course, it is the perfect raison d’etre for a villain or anti-hero. Whether, like Cable, they are planning to ‘kill Hitler as a baby’, or merely seeking their own power, we have a glorious gallery of archfiends from the rather lame Zarrko the Tomorrow Man to the Time Trapper or Immortus and Kang.
So as Age Of Ultron promises to draw to a close, we are given a solicitation for the Hulk where, as part of a super-secret section of S.H.I.E.L.D. (what’s the word for useless repetition?) goes on a number of missions because ‘History Is Breaking!”
No wonder, with the abuse the time stream has received recently. As I have stated before, Marvel’s strength has always been its adherence to its continuity. That is the very point that has always made the time travel stories so compelling. Now as we head into a What-If fest of alternate pasts, presents and futures on a scale that dwarfs the Exiles’ romps through the multiverse, I can only prey that this rash of time manipulations will come to a head and then fade again. Fade for several years as the dust settles. DC managed to play this right with both Crisis and Zero Hour on a minor scale, even if their build-ups resulted in partial reboots of the universe. Remember Booster Gold or the Team Titans? Like (hopefully) the All-New X-Men, in retrospect these titles made sense.
Internal narrative demands consistency. Logic. Rules. Fans demand continuity.
Break it at your peril.