Well, neither T28 or Tortoise got beyond prototypes because they really have 0 use. There were so many far more flexible ways of forcing though fortifications, so many disadvantages to a 100 tonne monster vehicle, and so many ways to deal with heavy armor. The ISU-152 was not a superheavy, it shared the standard soviet heavy chassis and was equal in weight to the IS and Panther.
Gamewise, the smallest component in the designer is a batallion (aka at least couple dozen vehicles). Also gamewise, you need to use the global exp to create and modify division templates. The US never envisaged more than 25 T28 at any point in time and even that was soon abandoned. So you would need to somehow balance having a tech tree and strategic exp spending for something that really has no role whatsoever. Yes, you could try to find a role for it, like anti-fortification (which was a lost cause, as the 105mm gun was just not going to do the job that required 210-mm howitzers).
If we want big shiny toys, why not add siege heavy artillery as a unit, like Br-4, Morser and larger guns, which were actually produced in significant numbers, and were actually used against fortifications?