So what could cause the comeback of tank variety in the near future?
MBTs are the result of an evolutionary process of development that went through tanks of various configurations. The fact that every army employing AFVs in this way has reached the same endpoint (MBTs) should tell you it's the optimal solution for what you are asking for (line of sight heavy cannon vehicles for general purpose warfare).
What would change that is specialization. But we already have that and you have rejected it.
in terms of tracked large cannon vehicles
The problem here is your emphasis on large cannon and frontline.
Tracked is a necessity for mobility of a large vehicle on general terrain.
The whole point of all the other vehicle sizes up to roughly the end of WW2 was that they deployed different levels of weapon and armor and speed. You don't always need a large cannon - it's overkill for many situations. So you got light tanks, medium tanks and heavy tanks (and so-called super heavies). The main difference is what they are expected to do, which brings us to your requirements.
If you insist on a large cannon on a vehicle then you get a large vehicle. Just for the sheer size and weight of a large cannon you need a large vehicle to house it. Then you need a big powerful engine to propel it, unless you are willing to risk less protection (armor).
You also need a fair turn of speed and there's no point in having a large cannon without a turret or the gun is basically useless on the frontline. Turrets have weight, lots of it, and need protection.
But you want a frontline "line of sight" vehicle. No can do.
To put an AFV with a large cannon in the frontline in line of sight with the enemy (who definitely will try and destroy it) you automatically need lots and lots of armor. You also need close support protection like machine guns to stop pesky infantry from getting close, because big cannon shells are big and you don't have a lot of room for them, so you can't fire them off just to keep infantry away from the tank. You also have multiple types of shell for different purposes, so you have to be careful about ammo use. Which means you need machine guns and they also need ammo which also takes up space and require an operator.
You also need some speed and power to maneuver on the front line and that also means you need a big honking engine.
But you cannot go too big in vehicle size or you tend to stick out like a sore thumb (and attract a lot of fire) and cannot e.g. deploy in "hull down" formation like a sensible tanker will want to. Very large vehicles also tend to have problems in transportation (even normal roads and railways can have problems, not to mention bridges - weight limits are important). So you cannot build a huge tank without many issues.
The list of requirements you have result in an MBT. It's why we're using MBTs in that role.
This will not change in a foreseeable future.
Note that MBTs don't work alone. You still need infantry support, other AFVs that perform specialist tasks (you cannot make an MBT that does everything) and air support (not to mention AA support). Sending MBTs in alone is worse than useless. So you'll still have the level of specialization we already have (again evolved over time to be optimal). This is what combined arms is all about. MBTs are one aspect of a balanced force on a battlefield.