The U.S. Army is planning to maintain its M-1 tanks (up to 7,000 of them) for another twenty years. There is no replacement in sight, and the chances of getting money for a replacement design are, for the near term, slight. The M-1 has already been in service for over two decades, and may become the first MBT (main battle tank) design to stay in service for half a century.
The electronics on the M-1 have undergone several upgrades so far, in addition to the larger main gun. More equipment has been added for urban warfare (an outside phone, cameras, reactive armor side panels, thermal sights and shields for the external machine-guns) and new ammo types for the main gun have been developed. A major enhancement was depleted uranium armor, which made the M-1 virtually invulnerable from the front.
The one remaining item in need of improvement is the 1500 horsepower gas turbine engine. Improvements here included electronic monitors on many engine components, an electronic logbook (to record all pertinent engine activity) and a maintenance program that makes the most of all this data. Because the engine is being monitored so closely and constantly, it's possible to carry out maintenance in a more timely (before something fails) manner. The army would also like to develop an improved (more efficient and less expensive to maintain) engine, but that is also a costly item they can't afford at the moment.
New anti-tank weapons are always being developed, and the army wants to at least be able to afford new gear to deal with new threats. One threat that is currently ignored is top attack warheads (that put a shape charge type attack against the thin top armor). There are also new types of mines and electronic threats. If the M-1 is to survive for half a century, it will have to evolve, as well as endure.