Putin using tactical nukes somewhere outside of Ukraine. It's a possibility if NATO's contributions make it too one-sided. I think we would see him test one on Russian territory first as part of the escalation.
However if Putin simply wants something and shakes the tactical nuclear button then he should be under no doubt of the response.
The issue here is someone wants to expand their current borders back to the last century, irrespective of the population that live there, then there is something wrong. If to him, all populations are territorial renters then every sovereign state is going to end up in nuclear brinkmanship.
The issue is that the guy's unblinking rehtoric does not seem to take into account the country but seems to suggest every scenario ends with Putin's tactical nuclear strike therefore give him what he wants.
Putin needs to know that a first strike - tactical or not - is just that. A first strike.
For Putin that seems to be that he owns the lives of those on his territory, thus taking a tactical nuke is something he can do. This promotes the chess moves - it involves the status of the attacked country.
1. An sovereign ex-soviet state is not part of NATO or Russia.
2. A first strike by Russia as a tactical weapon is not a first strike on NATO.
3. NATO can't 'retaliatory' nuclear tactical strike because adds more destruction to the sovereign state, nor can they retaliatory strike Russian territories as this is a first strike.
The point here is that the neutral status of the sovereign country allows the attack by Putin but doesn't allow the engage/retaliatory strikes. It's cold logic.
This is why Putin wants a buffer zone of those states and proceeds to throw his toys in the event they show the desire to join NATO/EU.
If those states joined NATO then any tactical nuke aggression would be a first strike by Putin on NATO, and the game of chess is checkmate. If Putin goes to nuclear then he simply imitates his own destruction.