Tank design is evolutionary. Arjun is an excellent tank with excellent features - a 120-millimeter gun, advanced composite armor, a 1,400-horsepower turbocharged engine, and advanced fire control and thermal sights. But many things can be improved.
The rifled gun is the easiest thing to change as it needs minimum modification, but will allow a much better range of ammo to be used. The Challenger tank also switched to a smoothbore quite recently. Since APFSDS will remain the dominant tank ammo against armour in the future, that is the direction to go. Once 130 mm guns enter production and becomes accepted, we may need to upgun Arjun as well.
Arjun has better armour than T90. But for a four man tank, this higher protection increases weight. Since better armour is always welcome, the only logical next step is a redesigned turret for a 3 man crewed tank with Lecrec like bustle mounted autoloader. The Ukrainians proposed a similar upgrade for Arjun sometime back. Germany and France had also proposed building such a tank with Lecrec turret on Leopard chassis. Eliminating manual loader will help in reducing tank height too.
T-90 has a significantly lower silhouette than Western tanks. Arjun has similar height to Abrams, Leopard and Challenger tanks. While lower silhouette provided significant advantage in past battles, it may not be that important in modern battles. No one is expecting mass tank battles today where tank destroying aircrafts, missiles and helos have proliferated. Instead, crew comfort is more important today - which leads to greater internal volume and tank height. Interestingly, Armatas are almost the same height as western tanks. So, it seems that is the direction to go.
While unmanned turret is the future, it is yet to prove itself in battle. Arjun hull is spacious and can be redesigned to accommodate the crew in a hull capsule. The turret can be further redesigned into an unmanned one once the technology is proven. Evolutionary design ensures that we can reuse the engine, transmission and other subsystems without rebuilding for each platform.
With Arjun, Abhay and recently showcased wheeled APC, we now have three base platforms and chassis to build a variety of vehicles for our armoured fleet.