In any superhero story these are pretty much a given, so we should expect (and hope for) this in CoT as well.
I only ask to implement it a little better than is usually done.
CoH had the Rikti invasion zone of course, but it really did not feel like an invasion that the heroes and villains had to go all out to repell. It never does because the logic of game development demands that the situation remains static,and worse, that encounters are scaled to what the player can handle. This removes the sense of scale from any invasion scenario.
Games about WW2 often do this a little better in their obligatory 'storming of Omaha beach' level, by adding a lot of action around the player so that even if they have to fight only a handfull of german soldiers, the scale of the entire event feels huge. As long as they do not overdo it and turn the player into the 'chosen one' who singlehandedly is responsible for the landing on the beach succeeding.
For multiplayer games this is not as easy to do, but I think we still can have a better approach that preserves the sense of scale and awe while not breaking the computers or the server.
First of all, while there are small skirmishes going on at all time, where groups of enemies test the defensese and either are repelled or make a temporary advance into hero held territory. These would be mini event like guild wars runs regularly in many zones. Player can join in for intangible rewards (like reputation) and a chance at a better than average reward.
At longer intervals and scheduled and announced ahead of time, there will be large scale events where the enemy tries to push through in a large scale battle. This gives players the time to prepare for this event. The event must run for a relatively long time (don't worry, I think I have a solution for the CoH Taskforces) and be scheduled for different time zones.
There must be a significant risk at failing the event, and this will have lasting consequences for the zone that may take days or weeks to be fixed in the following invasion events (that is, if the players 'lose' the scenario the zone becomes much more hostile and dangerous and a lot of amenities go away. The subsequent invasion event are not about defending or staging an offense, but rather about trying to regain the ground that was lost).
Players sign up for the invasion event either individually or as a group. In the first case they are assigned to groups by the game.
The event is structured like a story arc with many parallel missions. Each player group is assigned one of the many missions that is going on at any given time during the event, based of course on the make-up of the group. If it is tank heavy they are given a task to defend an important target or to front an incursion into enemy territory. A group that is stealth heavy gets infiltration missions against hardened enemy targets. And so on.\
Missions for which there is no group available are handled by game generated and run 'groups' of generic super soldiers. so that at any time the battle field is busy with conflict.
All these missions are run in parallel sometimes in a shared area of the zone, and depending on the mission in an isolated (by forcefields aka invisible walls) part of it to prevent bleed over from other groups. But it is important that players can /see and hear/ what is going on around them even if they cannot interfere directly, or cannot afford to be distracted by it. (after all, if you are part of a blaster group tasked with keeping enemy fliers away from the frontline as soon as you start shooting at enemies for another mission the game can immediately send in a wave of fliers and overwhelm you )
Regardless if a group succeeds or fails in their mission they are given a short time to recover (or they are 'rescued' by the game after failing badly). Depending on the overall success or failure rate of the groups the game sets up new missions as makes sense for the larger story of the invasion event. If most missions succeed the missions will be to press the advantage and the enemy will throw in more defensive units, or apply stealth strikes to stall the advance. And if players start losing they may have to try to defend their ground against increasing enemy attacks. And if one side made enough progress for the story of the event then the other side will start with scorched earth attacks and will start disrupting the lengthening supply lines, making it increasingly difficult for the 'winning' side to advance further.
When an equilibrium is reached the event is over, with the logic that both sides need to recover and build up new resources for another push. (and yes, I think that the 'regular' missions in the zone should be about supporting the war effort. Bringing in resources, spying out the enemy, sabotage, small targeted strikes at enemy strongholds that hinder their ability to build up forces for a next push).
Players can at any time between missions flag themselves as 'injured'. They will be evacuated to the field hospital near the start of the zone and from there they can leave the event. Either a new player or a temporary NPC will be assigned to the group so they can continue.
The developers would have to design dozens of relatively small scale missions with a simple goal for different types of groups, offensive or defensive, evacuation or air drop, stealth, artillery or tanks, against regular enemy troops, enemy heroes. THere could be giant monsters involved (requiring many groups to cooperate tot take it down) and machines of war or specially bred creatures to fight against. These would then be the building blocks of the story of the invasion event that is being told by the game, and influenced by the players. While not a trivial task, it is mostly building blocks and a system to combine those in a narrative whole that tells a unique story of a battle that should feel important to the players and that presents it at an awe inspiring scale. After all, the game easily can, and should !, run dozens of parallel missions staffed entirely by NPCs that are going on around the players at all time while they struggle to achieve their individual goal in the battle.