The T-100Z makes for a reasonable tier 6 choice.
The T-100Z is basically a KV-2 with a bonus 45 mm gun turret in the front. It even uses the same gun. The armour is the same as on the T-100, 60 mm, but that’s the price you pay if you want a little bonus firepower for plinking away at things your massive main gun can’t quite one-shot. Also, unlike a lot of tanks in this game, it saw combat! So historical!
Tier 7 would be populated by none other than the T-39! Yes, the lovable scamp that can start off with a similar 152 mm gun as its predecessor, but then upgrade to a whopping 4 107 mm guns.
Sadly, multi-turreted tank projects fall into several categories: early (huge guns, but pitiful armour), and late (slightly better armour, but terrible primary and secondary guns). KV-4s won’t be very interesting due to their 45 mms being pointless at such high tiers. So for tier 8-10, we’re going to have to start plucking out the weird ones.
Two tiers are covered by the VL (Vladimir Lenin project). The tanks consisted of two 76 mm gun turrets and one really big one: 130 mm B-13 or a 305 mm B-23 gun. The third project was only armed with one gun, so it’s not really interesting for the purposes of this tree. The tank was based on the experience of the Winter War, and on the rumours of new German superheavy tanks in 1940. Of course, it was deemed hilariously impractical and never built.
The tank was quite a monstrosity. At 260-460 tons (if you’re the kind of person that doesn’t like fun, there is a 320 tonn variant with a B-23 and only 45 mm guns), it only had up to 125 mm of armour, but boy, what an armament. A 130 mm gun wouldn’t be exactly amazing at tier 8, but the 305 mm gun would be pretty awesome. At tier 10, of course. The tier 9 hole would have to be plugged by something else.
That something else can only be the Ansaldo heavy tank project, which later inspired the T-39. In 1932, a 70-ton tank was designed for the USSR with a Char 2C-like electric motor, 30 kph top speed, and an impressive 152 mm or 203 mm main gun. Secondary guns included 76 and 45 mm turrets. The tank was never built, but its blueprints went on to inspire Soviet tank design for many years later, so odds are they are somewhere. Perhaps WG researchers will dig it up and reveal the wonders of the early super-heavy tank designs. Meanwhile, all we can do is dream.