Form follows function, Main Battle tank physical design more or less locked in the third generation at about the mid 1970's early 1980's. The hull has some features seen more on Eastern tanks of that era, the lines and contorts, head lights and flaps over the treads. But that's logical and not a major impediment. Its logical based on the Tanks pedigree, it's a evolution of Chinese tanks based on Russian tanks of a earlier era. The turret has some features i find unappealing, but complaining of it being "boxy" is not really one of them. Most modern tank turrets are boxy or faceted. If i were to complain about the look of that 99 it would say its to round.
now to the Armor, its long been said that the Chinese don't use Reactive armor plates on there tanks unless in time of war. That kinda makes some sense but i have my own opinion that is that the PLA actually uses modular bolt on armor. Not as a stand in but as they feel. It better suits there needs.
Now as to needing a 21st century tank the question is not about a new design but about the systems. The most state of the art tanks on the planet right now are the Turkish Atay, Korean K2, Japanese Type 10, Leopard 2A7+ add to that the Developing American M1E3 and Polish stealth tank concepts and compare features you end up with the critical wants.
Modular Armor. That is a armor system that is rapidly replaced in repair or upgraded as needed, as i just said I think the Chinese may have that.
Active defense systems, the PLA is rumored of having a hard kill system and the Laser Dazzler is a soft kill type.
Integrated defensive sensors and targeting array. It needs to see threats being launched and see what it wants to threaten. It seems to have those.
Signature reduction and mitigation sound and IR, well its still early even for western tanks. But the basic is gas grenades. As to Radar well its still early.
A good main gun preferably with a autoloader, Check.
a Automated Machine gun system, in the latest version?
NBC protection.
improved suspension, not always needed.
weight class medium to heavy. Type 10 is a weird outlier but most fit between 45-60 tons.
room for expansion, additional equipment that may become necessary in the future. China seems to have a new Type 99 series every 6 months that's because they are evolving there tanks.
now does the PLA need a new tank? Not really they seem to have the workings of a new tank already. They will continue evolutionary tank upgrades on the Type 99 series it seems to me. As it stands they probably are on par with the latest Russian made T90s in Indian service.