The more I think about it I think that if we can just get rid of 1UPT Civ VI has enormous potential. The building game is really fun...probably the best it's ever been. The governments are great, and religion is pretty good (but could be improved). 1UPT, however, has a few really grating implications that hold the game back.
- The AI can't handle it
- Civilian/foreign units can block mundane processes of building, settling, exploring in a way that is never fun
- 1UPT keeps armies small, which means that once an army is built (8-9 units) no more units need ever be built which means:
- There is not a whole lot of impetus to expand. Extra cities are a hassle, not a help
So, here's what I'm thinking. If we have unit stacking (say growing from 3 to 6 units per tile over the course of the game) then everyone will have to build far more units to hold choke-points, garrison cities, and have enough weight to launch an attack. This will mean having a larger manufacturing base will be a huge advantage, which is good: we need incentives for players and the AI to expand. More units should also mean deadlier combat, so the human has to keep building units in war (to say nothing of the need to build garrison forces as you advance and take new cities). As it currently sits it is easy to get through the entire game and only lose a tiny handful of units. This shouldn't be the case.
If we add military unit stacks, and allow unlimited stacking of civilian units and units of different nations not at war (i.e. you can have multiple full stacks from different nations on a single tile as long as none of those nations are at war with each other) then all of a sudden there will no longer be any roadblocks, ever. The only time units will be barred passage is by units in war-time or barbarians. The AI will be able to handle this much better, reducing turn times, and you'll never have to sit with a settler one tile away from where you want to found a city because an enemy missionary is just sitting there....
With this expansion all of a sudden becomes very appealing...perhaps too appealing. So, if we add a monetary cost to expansion, in addition to happiness penalties in captured cities, we have a nice balance. Yes, you can and should expand, but do so too quickly and you'll bankrupt yourself. You'll have a good reward for expansion (broader industrial/scientific/monetary base), with a more competent military opponent to stop your expansion, and a degree of friction in the form of maintenance which will force you to balance military expansion with having some way to pay for it (wars are very expensive, after all....a reality not reflected in the current game).
All of the other issues (warmonger penalty too high, roads are kinda pointless, etc. etc.) are minor and can be patched pretty easily. 1UPT, in my mind, is the only fundamental stumbling block that will hold Civ VI back from truly being great.