First, congrats on bringing one of the best games ever back to life. Especially the maps are stunning.
Secondly, I've read a few ideas concerning smoke, and why they're all requiring a large time investment (i.e. are not likely to happen). At the same time, smoke is such a powerful tool that it's probably the most important factor in a fight. I think there's one idea that should be not too complicated to implement and make the game slightly harder, but more realistic and interesting because of less predictability:
Currently the entire smoke cloud is treated as a single entity. All affected tiles are covered for three turns. Why not randomize smoke dissipation per tile?
- On the first turn you'd get the full benefit, just as now.
- On the second turn, each tile would have a small chance (perhaps 15%) of dissipation.
- On the third turn, each remaining smoke tile would have a large chance, say 70%.
- Those few tiles not dissipating after three turns would be covered for a forth turn.
One of the most useful tactics in UFOAI is stepping out of the smoke cloud to spot aliens, then have the rest of the gang, safely hidden in the cloud, open up with all guns. At enemies they can't even see themselves. Variable Smoke Persistence would make that tactic still possible, but suddenly very dangerous.