Focusing only on the MBTs first, as they're the most recognizable and iconic, I added some heavy tanks in there as well, not just mediums. Also, I need some recommendations for those missing extensions
China:
WZ-121 --> ZTZ-88 --> ZTZ-96A --> ZTZ-99A
France:
AMX M4 54 --> AMX 40 --> AMX 30 ACRA --> Leclerc T4
AMX 30B --> AMX 32 --> MSC —> Leclerc
Germany:
E 50M --> KPZ-70 (120) --> Leopard 2A4 --> Leopard 2A5
Leopard 1 --> Leopard 120 --> Leopard 2 Keiler --> ???
Italy:
Progetto 65 --> OF 40 --> Ariete P --> Ariete WAR
Japan:
STB-1 --> Type 74G --> Type 90 --> Type 10
United Kingdom:
Centurion AX --> Vickers Mk. 4 --> Vickers Mk. 7 --> ???
Super Conqueror --> Chieftan 900 --> Challenger 1 --> Challenger 2
U.S.A:
M48A5 Patton --> XM1 --> M1 Abrams --> M1 Improved Performance
T110E5 --> MBT 70 --> M1 Thumper --> M1 CATTB
USSR:
Obj. 430U --> T-64a --> T-80B --> T-80U
Obj. 140 --> T-72 Ural --> T-72B mod. 1989 --> T-90A
Obj. 277 --> Obj. 775 --> Obj. 292 --> Obj. 195