We're only stuck because nuke gives massive penalty, thus removing the penalty, downgrading the destructive power to balance and spread the effect out to other units would pretty much give us what we need, and none of it would even give any extra rule for players to remember.
I think it's moreso that people don't want constant nukes in their game about geopolitics -- it's a relatively rare occurrence IRL, most want the game to reflect this too. Its not obvious to me what you mean by spreading out the effect to other units, are we thinking we'd have a wider AoE area, but less damage per unit affected? I find the current function of nukes to be rather satisfying gameplay, gut reaction here is reluctance, though perhaps there's some use for this angle.
On that note, the wholesale destruction of entire units is a relatively rare occurrence as well. While I agree it could solve some congestion problems if we increase it's frequency, I don't think it would feel very satisfying to just have masses of units regularly being AoE one-shotted. At some point it's like why have conventional forces at all?
Re: the concept of stacking as a solution to congestion, I posit that the core problem of congestion is unit movement, not combat. Sure the combat bogs down when all plots become full -- is it this stalemate that people view as the problem? Imo its the tedium of the geometric puzzle that moving units around becomes, the waiting in interim tiles for paths to clear etc. Two evenly matched forces should become stalemated, this aspect makes sense to me -- the movement logjams not so much.
Some limited stacking for transit purposes only should be of little consequence to the existing gameplay paradigm. My proposed model may miss the mark but would encourage more thought on what can be done here. I'm not even necessarily a proponent of deviating from 1upt, but it strikes me that some kind of stacking would directly target the movement aspect of late game congestion, whereas other solutions floated thus far operate moreso in the periphery of the problem identified. Once movement is solved I'm not sure there's anything left to fix, really
they still have to find a way to make full use of all of their units (ex, the 1000 hit and run attacks per turn), but with the extra huddle of having to remember to not accidently stack units.
I think this is a bit of an overstatement about what would change -- the same series of decisions about having hit-and-run attacks before static attacks would occur, but player would have more choice about where to move the attack-and-move units afterwards. Similar to now, if you conduct the turn-ending attack too soon, your attack and move units cannot do their work in that plot -- but this is how things currently work, its no change. That said, it would be possible to focus hit-and-run attacks even more than currently, by keeping more units stacked closer to where the combat occurs, and this may be undesirable. Or it may not: being able to focus more attack and move attacks would be a mechanism to unstagnate frozen battle lines, and eliminate units entirely, freeing up even more space on the map. I don't see this stuff as a complexity issue so much as just balance-shifting, attack-and-move will be even more powerful than currently.
Ultimately my point is a 1UPT regime for combat, MUPT regime for movement may be both achievable and appropriate as solution to congestion effects