I'm going to make the controversial opinion and say invasion fatigue isn't even a real issue, it isn't a problem. People stop taking part in subsequent invasions because they have other interests they want to pursue and the amount of invasions seen in a short period of time result in a devaluing of them which causes them to place importance on things that matter more to them personally than as a faction.
And you know what? That's okay.
The toxic invasion fatigue that people act like is a big deal, that someone/a faction feel like they're being pressured by factions that have an out-of-character agenda to take away their cookies and replace them with
minor faction status salt is something that is largely fabricated by people who can't get behind the idea that individuals prioritize other threads over invasion #4 because they want to do something different for a little bit and not because they're hurting deep down because of another faction.
Do people like that exist, that actually feel like they're being overwhelmed? Sure. Is that a majority of the people who stop posting to invasions after the third consecutive one? No.
Are either of those problems that this will solve? lol, no. 4 invasions in 30 days, do you really think a major faction isn't going to see someone with this mandate or month or whatever going on and just wait to pile all of their invasions on them the moment they know they're in the clear to do so? If there's anything this will do, it's exacerbate the issue that a small group of people who currently are worn out from invasions in the feel-bad-camp and validate their point of view when this happens.
You're going to put a spotlight on factions that work around this and add scrutiny to the situation, and it's going to just get worse, not better. Yay, one month out of the year where a faction can't get invaded, or I guess 60 days for a mandate, and then we go back to our now condensed scheduled invasion season programming because now everyone who couldn't invade them will do it at the same time. Relentlessly.
Why? Because you put their stories on hold while you put your invasions on hold, making them resent you for your choice in hiding away from invasions, and then causing you to resent them for invading you right out of the gate (or whatever period of time later). We are already split into fake factions, let's not turn this into a "they're out to get us" split. I already lived through the One Sith vs Galactic Republic era of invasions, and I don't want to see people literally
hating each other because they're getting invaded again.
Invasions are like dominions, once you've done 3 you've done them all. Some people gravitate towards invasions, some people towards dominions and other non-PVP threads, and there's even more in the middle. When you get overexposure to either you want to drift away towards the other end of the spectrum, it isn't a
problem it is a reality. It is never going away.
Edit: that was a long-winded rant with no clear point, besides invasion fatigue being as inevitable as "dominion fatigue", so here's the tl;dr
- You won't get rid of invasion fatigue with this, even if implemented for every other month. Everyone will invade the moment they can. The less time you have to declare invasions, the closer together they will be spaced. You are making it worse by doing this.
- Doing this will influence those who already want to get away from invasions into actively disliking the writers who operate in factions invading them. If you want to divide people into us vs them, "they're trying to kill our faction and ruin our fun", this is both the perfect time and the perfect way to do so.
- People will notice invasions happening more often, and changes (if made) like this will lead people to believe that many invasions = bad; and seeing more invasions in shorter spans because of period where invasions can't even be held will cause people to see that as an issue, and one that requires additional changes (possibly suggestions for more moratoriums on invasions?) to fix rather than realizing that this is what introduces/exacerbates that issue.
- Invasion fatigue isn't bad. Factions and writers that act like it is bad are bad. No, I'm not sorry, but the idea that it's a bad thing to want to do something other than write a dominion/invasion/etc for the 7th time isn't a good thing, and it leads to this mentality that it's the invasion's fault and the only way to resolve it is by restricting them rather than telling that writer that it's fine and they should go and do something else since they probably aren't going to have that big of an impact on the invasion anyways if they aren't actually in the mood or headspace to want to take part in one anyway.
Valiens Nantaris
Ellie Mors
, Despoiler Carnifex , and
Ronan Vizsla
, and many others on the board have all brought up good points about the map game and making things more dynamic. Most of these ideas being a great way to make the map game more fun while being less serious. 
