I'd like to open this thread to discuss the upgrades for all tanks. Not to discuss buffs or nerfs tanks might need.
The two main lines you can go with the tanks I looked at so far are either go for close combat and be stronger against infantry or strengthen you AT abilities. It probably makes sense to go fully either way. With the current meta I clearly go for long range shooting as there's little to win in close combat with infantry.
Some options aren't clearly specified. The option I'm currently thinking about is whether to get the skirts for the Panzer IV or the repair kit. It doesn't tell how much better the skirts protect the tank so hard to compare benefit.