Personally, I see these modifiers acting as a foil to the various mandates that are already influencing our Invasions. While, yes, they do have a small chance of happening now and again, it definitely adds a fresh, and exciting mechanic to the fold.
For example, a Faction that has the Defensive Stronghold mandate can have unlimited allies, against the attacking major faction - who only has the five friendly accounts it can bring with them. You could literally have over 3/4
(More or less, and Faction depending) of this site's population arrayed against the attacking Faction, and as it stands - there's nothing that can be done about it. If the attacking faction manages to win against such a stacked deck, they get One hex on the map and have to do it all over again. Yes, there are inherent weaknesses with having so many warm bodies to throw at the opposition, but there's little reward for those that manage to triumph over such unbelievable odds.
Having the chance to nab Two extra hexes, when pitted against so many people? Makes that whole stressful ordeal feel like it was worth the hair-pulling, at least from the attacker's perspective. Heck, if that's too much for some folks - why not alter it so only One extra hex is gained, instead of the two?
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With that being said, I do echo some points that were brought up previously and would like to see some modifiers tossed at the Dominions as well; adding some extra flavour, rules-wise, onto what each and every Major Faction's already able to come up with. I can't think of any right now, but perhaps something that's in line with the Rebellion and Invasion modifiers would do.