I suspect that the next US MBT will have a gun/launcher like the old Sheridan light tank issued to the 82d back in the 60's. But some new kind of armor would be necessary for survivability.
The next US tank may simply have new ammunition. This has been worked on as far back as the 1960's (the gun/launcher of the M 551), but there was some very interesting work in the late 1980's, such as TERM (Tank Extended Range Munitions), a form of top attack round fried from the main cannon, and X-Rod, which was rocket propelled throughout it's flight to achieve higher impact velocities. Though tube missiles were developed by Israel and Russia as well.
TERM was eventually dropped by the US, but the ROK developed that idea and it is issues as K-STAM (Korean Smart Top Attack Munition), which has an effective range of 8km. LAHAT, the Israeli through tube missile has a maximum range of 13km, although it would need some sort of forward observer. Future tanks could be tied into some sort of network, or fly their own quad copters ahead to scout for targets.
As for armour protection, the Russian Armata has put into practice ideas which were floated as far back as the 1980's as well. While creating a modern tank seems to be on the back burner in the West, rearming existing tanks with new ammunition would go a long way without breaking the bank or running into mobility and other issues.