Depends on the tier really.
Firstly, it is worth mentioning the sort of rock-paper-scissors idea that is present at lower tiers.
Medium Tanks > Light Tanks > Tank Destroyers > Heavy Tanks > Medium Tanks
Ideally, the “perfect” map would accommodate each type of tank equally, but there is so much overlap in some aspects that it might just be outright impossible.
That being said, for lower tier maps (1.0 - 7.0, or in time period up until ~1960) you need to have a mixed CQB-long range map with more emphasis on the CQB, due to the massive prevalence of heavy tanks, which are extremely difficult to deal with at range but can still dish out damage. While that is the point of heavy tanks in general, as a gameplay mechanic it would be unfun for everyone on the other side of said heavy tank. Picture it as trying to damage a car with a stapler from 1km away, it just won’t work. So, to “counter” said heavy tanks, maps need to have some form of flank route(s) in the map that are accessible to both teams, so that it isn’t a cakewalk for one team or the other to just setup on a corner and snipe the entire other team.
However, designing maps in this way sets a sort of “barrier to entry” to anything that doesn’t have enough armor to stand up in a frontal fight, which means tank destroyers, light tanks and, to a point, even medium tanks, would be glorified piñatas for the heavy tanks. So the map would have to also be designed to incentivize/reward playing the riskier tanks, maybe with flank routes or sniping spots or just having an SL/RP bonus to playing them (which isn’t part of map design, but still worth mentioning).
A good example, in my opinion, of this sort of design is Advance to the Rhine. It mixes CQB and at least a semblance of long range pretty well, allowing for both playstyles and most vehicles to be effective. Imo if the sightlines were a bit longer or the map an extra 50m bigger (in all directions) it would be great, and it is already a pretty good map. A bad example is Finland, being just three lanes with little to no movement between them, and not much room for light tanks or tank destroyers to be effective in.
For top tier, (7.3+) range is the name of the game. Combine extremely accurate guns with extremely powerful rounds and you get the potential for most hits on any target to be devastating. So being at a range where getting those hits is actually somewhat of a challenge is ideal, to allow for avoidance of being hit (by changing speed/direction, etc.) and also allow for sniping.
The problem here is that having one side be able to control a powerful sniper’s nest style location can effectively lock down that area of the map from one side, potentially even denying the opposing team a spawn point (or multiple in bad cases). Assuming equal matchups, this would be countered by going a different direction, but on some maps (European Province, Poland, etc.) that just isn’t possible due to the sniper’s nest area providing an overlook of the entire map. So that would need the map to be hilly to provide defilades for tanks to hide in to approach enemies, and allow support vehicles like IFVs to be able to “ambush” enemy MBTs. However, if you make the map too hilly, either with hills too steep or the top of the hills being too open, or various other issues, you’d end up with just another “lane” map where everyone is directed down a very small area to avoid being shot at by the other team, turning the game into a massive CQB battle when both sides get nearer to each other. Think of Arctic Base or whatever it’s called, practically no one goes up the “trench” hills because you risk being shot from basically the opposite spawn. That forces everyone down the “trenches” to the cap points to stay in cover, which while a good thing is also forcing CQB encounters that not every vehicle at top tier can handle.
A good map for top tier, in its current state, is the new large version of Tunisia. It has long open sightlines with enough hills to safely hide behind practically anywhere on the map, along with a small (but not insignificant) CQB area in the form of the center of the map and the towns on the edges.
In general, what to avoid is the three-lane map design similar to those found in FPS games. There needs to be ways to move between the primary points of contact easy, to both encourage different playstyles but also to avoid stagnation.
Another thing that may be a consideration in map design is the cap points. Generally, the cap points in this game are just CQB fests at all tiers. For top tier especially, a switch to a mode like Battle where you have to “push” into where the enemy is waiting while they simultaneously do the same elsewhere is the kind of style of game you would want. It combines range (the waiting) with CQB (the pushing) pretty decently, and implemented well on maps with cover, and also probably moving the cap away from the spawn in some capacity, would make for an interesting game for both the attackers and the defenders of any given area. It would also encourage teamwork in a way, because if the enemy team is breaking through a certain area of the map, it would be prudent to have a few friendlies go over there to “hold down the fort.” Obviously, describing map design in text is hard, especially when the maps are for a very niche game style that is hard to balance around.
