About the only way to protect a tank fully is to either go for a motorized fortress (British tanks since the Cheiftan and the Merkava follow this principle, and the M1 and Leopard 2 are not far behind), try not to be hit (light tanks like the AMX-13 or CV 90120 would need to do this), apply some techno magic (Chobahm, Blazer reactive armour, Drozd or Arena active systems, electric armour) or go for exotic designs (S tank).
Every approach has some advantages and some disadvantages, and there are lots of threads on this forum which discuss them. The point that I am making is that we need to know what it is we want to do with the Forces before we can decide on an approach to the tank/DFSV/assault gun or whatever. If our forces are configured to fight a conventional battle, then all out MBT's are the way to go. If we are looking at a primarily "first responder" expeditionary force, then light tanks are the way to go. If we are seduced by the siren's song of a "transformational force", then techno magic needs to be applied.
In the real world, trade offs and compromises are needed since many of these factors and others are in play; we may "want" MBT's but have no practical way of deploying them or supporting them in the field. From a forcasting point of view, I will still stake out a position for a smaller, lighter tank to satisfy logistical and mobility issues for a deployed force, and note that clever design, such as placing the crew in an armoured "pod" or surrounding them with vehicle mass such as the engine block and fuel tanks provides a great deal of protection without increasing the overall mass of the vehicle to unreasonable levels. A developed version of the CV 90120 would be in the 30-40 ton range, while something developed from the Puma IFV would be in the 40-50 ton range, which should satisfy the logistics and mobility issues. (A LeClerc is in the low 50 ton range, but cannot be spun off into a family of vehicles like the CV-90 or Puma. If we are willing to accept that limitation, it represents the "upper boundary" that I would set for our future tank).
Since we are dealing with an uncertain environment, and know that technological change will be constant through the lifespan of any future vehicle, we should also look at making it as versatile as possible, so high angle gun mounts, the potential to fire smart munitions, high bandwidth communications and sensor systems should all be considered as desirable for any new design.
