Disclaimer: this article contains only the historically justified reserves for buffing the T-28. I do not necessarily believe that any of these buffs are possible, likely, or necessary, either individually or put together. Buff My Tank segments are for entertainment purposes only.
The T-28 is one of the older tanks in this game, both by development time (most of the world was still on FT-17 clones in 1932), and by being in one of the first tech trees that were introduced. Through its long service in the Red Army, it was continuously modernized, but the T-28 in the game has remained the same (aside from losing its 85 mm gun). Let's see if we can apply some modernizations to the in-game tank.
Probably the most well known modification of the T-28 is the T-28E (ekranirovanniy, with screens). These tanks received additional armour plates, welded on to the existing armour. Instead of the in-game 30 in the front and 20 on the side, which even the weakest gun in the game has no problems with, the armour would grow to a respectable 80 in the front and 40 on the sides. This amount of armour actually matters, and will not be easily penetrated by tier 1 guns anymore.
Various T-28s were developed with increased speed. The T-29, which was a T-28 with a Christie suspension, could reach 57 kph. The T-29-4 could reach 72 kph. The T-29-5 could reach a blazing 81 kph.
In 1935, a hydromechanical transmission was developed for the T-28. Such a transmission would boost the agility of the tank (see the long-lost PzIV Hydro).
The tank was also modernized in 1935, into the T-28A. The T-28A could reach 68 kph (compare to the 45 kph the in-game T-28 is capable of) on the same suspension type.
I previously mentioned an 85 mm gun. Some of you, if you have been playing long enough, will remember the F-30 85 mm gun on the T-28. However, that is old news. The T-28 also saw tests of the F-39 95 mm gun. The ballistics of the gun were to be the same as the 95 mm divisional gun F-28. However, work on the 95 mm guns ceased in favour of 107 mm guns, and the F-39 didn't make it to mass production. This doesn't stop us in World of Tanks, however, so let's keep speculating. The F-39 gun was 39 calibers in length, which, although not a lot, got the shell to 650 m/s. It was estimated that the shell would penetrate 65 mm of armour angled at 60 degrees from a distance of 1000 meters. In similar estimates, the F-32 gets 43 mm of penetration under those conditions. Assuming the ratios hold, the penetration of the F-39 would be approximately 100 mm at 100 meters, against flat armour (given the in-game penetration of 67 mm for the F-32 gun). This is still lower than the ZiS-4, which is currently the top gun on the T-28, and still lower than the F-30 85 mm gun (that made its way to the KV-1). A F-39 gun with a lower much lower ROF, but much greater damage, wouldn't be overpowered for the T-28.
Here we go, an 80 mm armoured beast, flying around at 81 kph, with a 95 mm gun :) That ought to make the T-28 not suck.
Popov et al, Builder of War Machines, Lenizdat, 1988, pp 38-52
Solyankin et al, Soviet Medium Tanks 1924-1941, Zeughaus, 2007, pp 32-41