Hello and welcome to “Buff my tank!”

The “Buff my tank!” articles are meant as an historical way to look at some tanks considered underpowered in game and ways to improve their combat abilities discussed by the original German engineers.
Beware that while being sometimes ironic in tone, the article treats about both costs and benefits of every choice and it most likely will never be listened by WG as suggestion.

Often forgotten and underestimated in the shadow of its mightier successors, the humble Panzer I was mainly meant to be a training tank but ended up being a pretty important part of the early Polish and French blitzkrieg campaigns.

During its service history several modifications and evolutions were developed and proposed, sometimes greatly changing the tank’s role far away from the puny vehicle that didn’t exactly shine during the Spanish civil war and versus later opponents.
For this article we will use “Panzers I and II and their variants by Spielberger, “Panzer Tracts 1-1” and “Panzer Tracts 1-2” by Jentz & Doyle

 Firepower

The staring projected armament of two MG13 clearly designated the tank to be used mainly against infantry and soft skinned vehicles as their penetration was too lackluster against most armored vehicles.

During the Spanish civil war the 20mm Breda auto-cannon was installed, giving it some punch against similar light tanks.
The VK 6.01(ausf C) upgrade brought the EW 141 MG that allowed better penetration, although using precious Tungsten.

Protection

A measly 13mm all-around protection was enough to protect against SMK, but anything heavier would pierce it easily. Bolt-on 15mm plates were often added, giving it decent protection against weaker auto-cannons and AT rifles.

Panzer I ausf C brought frontal armor to 30mm (at the price of bringing weight to 8 tons), although it was the VK 18.01 (ausf F) that in December 1939 brought a serious upgrade to 80mm frontal and 60mm side armor, though at the price of bringing the weight to 18-19 tons with evident effects on mobility (a top speed of 25 KM/H was planned):

The Krupp M 305 engine at 60HP was nothing to write home about although at 11 HP/ton it still allowed a maximum speed of 40KM/H for the ausf A on roads.
One of the very first proposed upgrades was the 80HP Krupp V-8 engines during design phase in 1932, while also of note is the 85HP air-cooled Krawa engine tested in 1934/35.

In 1935 the Maybach NL 38 was added, which at 100HP was a definite improvement in mobility.
In September 1939 the 150HP Maybach HL 45p was to be installed in the ausf C, giving the vehicle a top speed of 65km/h (18,75 HP/ton) although the drive train was rated for an extreme 81 KM/H.

An even more extreme upgrade was planned in 1942 for the VK 6.02, installing a 200HP Maybach HL 50 engine which would have given an amazing speed of 80 KM/H with a power to weight ratio of around 25 HP/ton.

Conclusions:

Although a viable design in 1939, Panzer I was always quite under-gunned and modifications given to keep it in par with competition never brought it above decent.
WOT already shows most upgrades (aside from the Ausf F/ VK 18.01-02 which could be an interesting tier IV if used with limited match making).

All in all: No further buffs possible unless using a pretty much new design