It makes no sense to cap the survivors at 3 from the lore perspective. I get the point game wise. instead why not combine the point of attack and defense concepts.
Automated turrets and the huntress trap already deal with aggro. So the core of the mechanic I am suggesting is already there.
Why not have points on the map have something like a tower. and you can deploy one or two survivors on it. Like turrets these are invulnerable. but shoot enemies.
while deployed their health is added to the attack team (the one you control) and the defense survivors continue to deploy their skills.
This would allow the defense team to increase the attack groups power, while maintaining its limitations (hitbox, etc). their buffs and skills would be the only things added. they are literally turrets on specific points on the map.
This also gives the option of the player to head to the safe zone if things get to heated. thus allowing for a growing scale of power.
Add to that if we can also level up the defense team, the levels needed would easily pass 100 and you still wouldn't be done.
If we add the possibility of changing the people deployed on the towers, it makes the game have a interesting dynamic.
some survivors can be built as towers (Tank, Huntress, Engineer) while others can be built as the attack group, (Swat, Ninja, Medic).
the variability of tactics give a level of planning and consideration when choosing your defense and attack survivors. (It also makes the game a whole lot of fun to max everyone while not dying).
It can be for specific game modes like liberation of a city. and works really well in endless where the enemy evolves at the frithening pace.
HP is the biggest thing in endless, and no matter how hard one tries you will only get around 2000 HP, but add 3 more survivors and you would easily get to 3000. This among others would keep the game modes from being to repetitive.