The only paths for the Soviet Union I've found are Yezhov's and Tukhachevsky's. When doing the Great Purge focus, a series of events will happen. The first series of events is mainly Tukhachevsky's coup. Pick the decisions that ignore dealing with it or allow it to be more successful. If you want it to fail, choose the -100 pp decision and he will fail and die. Yezhov will takeover by default at that point. The decisions don't matter as far as I'm aware. Depending on the choices taken, Yezhov either kills Stalin quicker or after the Great Purge focus is over.

I'm unsure how Trotsky or Bukharin are supposed to takeover. Trotsky needs to be "invited" and Bukharin needs to be chosen as the next "General-Secretary" or leader. When it came time to choose which path to take, Tukhachevsky as a dictator was the only option for the event so there's something I'm missing.

I'll find out more in a bit.

After some more testing, After Tukhachevsky's successful coup, there's two national focuses before the "Marshal's Decision" that concern the NKVD and the Millitary.

If you keep the NKVD and the Military as is (choose the +50 pp options for both) you get Tukhachevsky as dictator.

If you kill the NKVD and keep the Military intact, you get Bukharin.

If you keep the NKVD intact and kill the Military, you get Trotsky.

And if you kill the NKVD and kill the Military, you get Bukharin anyway.

Probably better to keep the Military with Bukharin instead of killing both for the sake of it.