In a single-player, is there any reason that having the bubble be the entire world is a problem? I am perfectly fine with someone who is not present for a fight not being able to participate in that fight - this is the risk you take of splitting up. It seems ludicrous to me that you should have multiple time streams so that one character can run clear across the world in six seconds.
I recognize that there is a complication in multi-player games, where if you are doing different things in different areas, forcing someone into turn-based time because of something that is happening far away could be boring for that player.
One option for multi-player: give players who aren't involved in the combat due to distance the option to join initiative or not. If they join the initiative, they're locked into turn-based mode, but could theoretically participate in the combat. If they don't join the initiative, they can continue as usual, but are unable to enter a large bubble around the combat or affect the participants in any way until the battle is concluded. So characters think they are close enough to get there in time could join initiative and try to run, and characters that are really far away could continue with whatever they're doing, but could not participate under the battle under any circumstances.
