Hi Serban. Personally, I have trouble imagining what the conditions inside a separate "bubble of time" could be other than being a part of a parallel universe. Would this bubble, like our own, feel that it had its own timeline of history stretching back to our shared big bang? Would it continue to have its own timeline stretching into the future which would continue to be selected for each of its occupants by choice, chance, and the actions of others? Or would its "bubble state" mean that it was somehow more limited in its characteristics than that?
By imagining a "bubble of time", I believe you are imagining exactly what I am imagining - a separate timeline, decoherent with and separate from our own, but with its own feeling of completeness. All grandfather-type paradoxes are resolved, because each choice, no matter how odious, merely starts down a new timeline which cannot have any effect on the original timeline where that choice was not made.
Since time for us is limited in a way that doesn't appear to allow for us to experience any more than one timeline at a time, then what I am proposing gives us a space for each of those uniquely limited timelines to exist within:
a fifth dimension of possible branches which, like a line drawn on a mobius strip, from the fourth dimension appears to feel like a straight line as it twists and turns in the dimension above, and
a sixth dimension which represents the lines which there is no way for us to directly branch to - like the universe where a quirk of evolution has caused humans to still have their tails, for instance.
Your way of imagining would still appear to have the same result as mine: that's because here in the fourth dimension, we remain unaware of the dimensions above. Likewise, your "bubble" would also be unaware of and be unable to affect any changes in the universe it had bubbled off from, and would merely feel like it was continuing to travel upon its own unique line from past to future. But to imagine that without also imagining that there must be a "multiverse" of other parallel universes, as you suggest, still feels to me like it creates other paradoxes. This is why I have become such a big fan of Everett's Many Worlds Theory.
Finally, here's what I think about the question of "why haven't we been visited by time travellers?", which I think is very similar to the question of "why haven't we been visited by aliens?". Let's take the position, first of all, that those things haven't happened, because there are of course some who believe that they actually have, and perhaps that our survival this far has been because of the benevolent interference of higher powers, be that time travellers, aliens, or a spiritual force like God.
The universe we are in is already unfathomably, astronomically, statistically unlikely to have happened. How did we make it this far? If we are to imagine the potentially catastrophic effects that alien conquest or foolish time travellers might have on our universe, then is there some way that we can imagine that all those catastrophes do happen, but that the quantum observers who are creating en masse the consensual reality in which we are participating have chosen (using their higher dimensional ability to see possible outcomes) to not travel down those paths? This must also mean there are parallel universes where atomic warfare destroyed civilization fifty years ago, or where the Y2K predictions turned out to be completely accurate. All those other universes exist, they're just not the ones you and I have chosen to observe: but other most likely did. How's that for a brain buster?
Thanks for writing!