When substantial action follows the skipped player, he will have a live hand but can take no aggressive action.
Ultimately, I think it will be more practical to leave it to the discretion of the TD to determine whether the fairest decision will be to allow the skipped player to act with all options available, allow the skipped player to act but not take aggressive action, or to have the hand killed. I can envision situations in which any of these rulings would, in my mind, be the fairest solution.
I am fine with using the minimum actions in the definition of substantial action as repreesenting a first "goal post"... If substantial action has just occurred, in most cases, the skipped player would lose his right to take aggressive action unless the out of turn actions occurred so quickly that it would be unfair to penalize the skipped player.
But, as Tristan suggests, I agree that we should be open to the possibility of a harsher outcome (e.g. Killing the hand) if the action has gone significantly past that first goal post. I do not favor a set rule that guarantees the skipped player's hand will be live. For example, I think there is a big difference if there has been a call-fold and the skipped player speaks up, and if there has been six subsequent folds before the skipped player speaks up, despite the fact that both situations would constitute substantial action and the dealer has still not dealt the next street in both cases.
Substantial action PLUS further delay must be met with more serious consequences for the skipped player; even if the skipped player claims he did not intentionally allow the OOT actions to occur, at some point, as more and more actions beyond the initial goal post occur, his excuse loses more and more credibility.
In summary, it may be that any new rule will need to come about from a combination of the approaches that we have discussed. However, I don't think that a set rule that keeps the hand live based solely on whether substantial action has or has not occurred is a complete solution.