That's an interesting question.
I think the definition, as it stands, is already quite broad. I think that reflects the concerns of opposition MPs, the Canadian Civil Liberties Association and the Canadian Bar Association when the act was first introduced.
The definition of “threat to the security of Canada” is intentionally broad and somewhat vague, because CSIS is an agency that needs to be able to interpret potential threats, things that are not quite there yet and things that are hard to discern. Having very firm lines around what is and is not up for a potential investigation, as in the context of a criminal investigation, is something that is avoided by having more broad and vague language, as is used in the CSIS Act. It's not so vague as to not denote anything, but it is quite vague.
To say how broadly we can interpret it.... I have a hard time with any broadening of it, especially when we talk about paragraph (c) under the definition of “threats to the security of Canada” in section 2. What we're talking about here is terrorism and violent extremism. These things are already quite hard to put your hands around, to understand what is or isn't violent extremism, for example. To say that as we currently understand it we could interpret it even more broadly, I don't see where there is a limit. I think we have to rely on the text.
Where there is discretion is in that weighing of the factors to decide whether or not that definition is met. It's not what the words themselves mean.