M60/AX "Super 60";
the M60A3 for the Twilight War.
The M60A3 in US service was declared surplus to requirements in 1991. A steady wind-down of stocks occurred but by the time hostilities are discernable it's sensible that the Department of Defence would put the brakes on discarding these vehicles.
General Dynamics had put out a serious upgrade package for the M60 in 1985 that was turned down because the US was moving towards M1. However it is to be noted that this package was superior to pretty much every other package available and was a pretty much bolt-on upgrade. I'm just going to cut-and paste the wikipedia entry:
"The tank upgrade is based on the M60A1 RISE hull and the T95E7 turret as used on the M60A1 and A3 variants of the M60 series. Mobility was increased by 20% with a new engine and transmission. It featured the AVCR-1790-1B engine coupled to a Renk RK-304 transmission with 4 forward and 4 reverse gears. The torsion bar suspension system of the M60 was replaced with the National Waterlift hydropneumatic suspension system (HSS).[101] Survivability was enhanced with a layer of Chobham spaced applique armor built around the M60A1 turret, that noticeably changed its appearance. The hull armor is enhanced with a layer of laminated steel armor panels covering the frontal arc of the hull. A pair of steel track skirts were added as well as Kevlar spall liners for the fighting compartment. It has a crew of 4, the commander, loader and gunner are positioned in the turret and the driver in the front of the hull.
The weapons of the M60/AX are similar to those of the M60A3, but different models were used. The main gun is the rifled 105 mm/L55 M68A1E2 with a longer XM24 tube and a thermal sleeve, the same weapon used on the M1 and M1IP versions of the M1 Abrams MBT with 43 rounds.[102] The 7.62 mm M73 coaxial machine gun used on the M60A1 is replaced with a 7.62 mm M240C, with the same number of rounds. The M19 cupola was replaced with a low silhouette model with a pop-up hatch for the commander and a 12.7 mm M2HB machine gun on a pintle mount with 600 rounds. The Fire Control System (FCS) is essentially the same as used on the M60A3TTS consisting of an M21E1 solid-state ballistic computer, Raytheon AN/VSG2 Tank Thermal Sight (TTS) for the gunner, a Raytheon AN/VVS2 flash-lamp pumped ruby-laser based range finder, accurate up to 5000 meters, an M10A2E3 electro-mechanical ballistic drive and solid-state analog data card bus. The prototype built did not have an optical range finder but one could have been easily installed.
As one of the first upgrade packages offered for the M60 series, the M60/AX prototype demonstrated the potential for upgrading the M60A1/A3 and even the M48 series as well. Even though this update package offered M60 users an opportunity to dramatically increase the combat capabilities of their tank fleets, no country ever bought the update, and the program effectively ceased by the end of the Cold War. Only one prototype was built. The overall failure of the Super 60 Program was likely due to the lack of immediate necessity for such a vehicle."
While it's noted that this vehicle became the Israeli "Magach" it was actually a superior vehicle due to General Dynamic's superior manufacturing ability.
Note that the Super M60 as depicted is the "demonstrator model". Super M60s in the Twilight War would probably under go similar modifications the M1A1 underwent such as the TUSK program. Importantly there's no need for any design work to be done, the upgrade is ten years old at the start of the war. The sensors would probably be upgraded to M1A1 standards and incorporate other technology that had arisen during the time of its design and the start of the war.
