From what I understand you're suggesting a mode where the game doesn't force-end when the game has been going for too long? I kind of disagree if that's the case, since sometimes the defence can be nearly impenetrable. If you place enough Ice, Quake, Archer IV's path 2 and Zeus IV path 2, Turrets III path 1 and ocassionally some sorcerer/artillery towers your base can take on pretty much anything that gets thrown at it (Depends on the map as well of course) unless you send a freight train of level V slimes combinated with witches and/or blazes which you would need about 3 or 4 people and tons of coins for. Such an attack plan is rarely being brought into practice, since there always will be people on your team that are new to the game and you're not always with 3 or 4 people.
As far as the modes go, we already have different varities of modes you can select in Tower Defence, so I don't really think that's needed.
