The PzIV was a very old design. Development began all the way back in 1935, and the overall layout was even older, dating back to a Carden-Loyd design from 1927. The original specifications were for a 17-18 ton vehicle with very light armour (barely enough to resist low velocity 37 mm guns and small arms). Krupp tried to introduce a novelty with this tank, torsion bars, but due to poor performance on their prototype, they fell back to good old leaf springs that have worked perfectly fine so far.
As the war went on, the tank was modified more and more: longer 75 mm gun, applique armour (then, thicker armour plates), spaced armour, bigger engine. During all this time, one thing didn't change: the suspension. Turns out you can't slap more armour onto a tank forever; even when increasing the front armour to 80 mm, the turret front remained 50 mm thick. The tank's mass inflated from 18 tons to 25.
In an attempt to make the PzIV more Panther-like, sloping of the front armour was proposed by Krupp for the Ausf. H design (you sometimes see this referred to as Ausf. K or Ausf. L, but that is not the correct name for the tank). However, even something as seemingly simple as combining the front of the turret platform and the armour plate above the transmission into one sloped plate would require a drastic redesign, as it would effectively make the suspension and brake drums impossible to access in the current form.
Another attempt was to boost the firepower of the tank by installing the Schmalturm turret carrying the 75 mm KwK 42 L/70 with a much longer barrel than the PzIV's KwK 40. If you look at Doyle's line drawing of the tank, you'll see that the wheels on the front bogey on each side are all steel. This is because this upgrade overloaded the front road wheels and would strip the rubber from them. As you can imagine, this wasn't great for the reliability of the spring elements, either.
The Panzer IV/70 tank destroyer did fit the longer KwK 42 gun on the PzIV chassis, but did so in a fixed casemate. Even so, the increase in weight was enough to have to strengthen the two front wheels in the same way. The Alkett version of the design had to do this to the front two bogeys on each side, or half of the road wheels on the tank.
There was another proposal, the PzIII/IV mit Einheitsfahrgestell, a tank based on the PzIII and PzIV designs, featuring sloped armour. However, it was still stuck with the same old KwK 40, and would require a new suspension to be built. Despite using many existing components, this was effectively a new tank.
As you can see, the chassis was at its limit. Trying to add any features would either overload the chassis and have a very poor effect on reliability or be so complicated to implement that designing a new tank would have been easier.
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