I'm a little surprised you haven't gotten an answer yet, because the Soviets were in fact responsible for some of the most innovative and impressive doctrinal developments of WW2 — with the caveat that I wouldn't quite call them "tactical", because perhaps the most important thing they did was realize that there is a level of military action between the tactical and the strategic: the operational.

To understand what exactly operational art was, and why it was so groundbreaking, I would point you to this quality answer by u/dagaboy, which discusses it in the context of the Normandy landings.

Meanwhile, the actual strategies, operations, and tactics that the Soviets developed with this understanding were known as "Deep Battle" or "Deep Operations". In general, Deep Battle was much more revolutionary than anything the Germans did or theorized during the Nazi regime; the much-mythicized Blitzkrieg was, in fact, not really articulated as a doctrine at all, and wasn't all that much of an innovation on what had been standard practice for decades anyways.

Strangely, I'm having a lot of trouble finding answers on this subreddit that actually discuss the development and practice of Deep Battle in any depth, with the possible exceptions of this answer on Soviet paratroopers and this one on why Soviet generals were considered so good, both by u/Jon_Beveryman, and this answer on how German doctrine was actually not all that innovative or interesting by u/wotan_weevil.