I will have to disagree with George about merging tanks with NLOS weapons. This is already happening, for example Russian through tube missiles, the Israeli LAHAT through tube missile and the ROK's K-STAM top attack round. Many of these systems can actually be traced back conceptually to the Americans, for example the 1960 era MGM-51 Shillelagh anti-tank missile system, or the 1980 era TERM (Tank Extended Range Munitions) program.
The tank provides a powerful, heavily armoured and mobile fire support platform, while the combination of extended range and NLOS munitions combined with a high fidelity situational awareness allows the vehicles to engage targets in places and situations where conventional tanks cannot. A simple example is tanks in defilade cannot engage with APDFSF rounds, and using indirect fire with unguided HE or HEAT is inefficient and a waste of ammunition. Since a Korean K-2 can engage targets out to 8Km with K-STAM rounds, and Merkavas can engage at up to 13Km, the commander can use his armoured forces to shape the battlespace from a much greater distance. Since the firing platforms are tanks, they can also rapidly move to exploit the shaping, or retire if unsuccessful (more so than conventional or even SP artillery, for example).
Probably the greatest challenge for future tank designers will be how to get 40+ rounds into the tank in safe and efficient storage, but also easily accessible to the crew to engage a wide range of targets in a rapidly changing battlefield environment? If you can't carry sufficient main gun rounds, are there other ways to deal with this? I'll throw out one historic example: the Swedish Stridsvagn 2000 T140/40 was designed around a 140mm cannon to engage tanks, but carried a 40mm coaxial cannon to engage any lesser target (the thought being a 40mm would be able to deal with IFV's, bunkers etc. that presented themselves). Obviously there are other possible solutions to the problem, so it should be interesting to see how far "out of the box" new tank designers will go.