===> Additional adjacency bonus for theater districts
They could move the +2 city center adjacency bonus that the acropolis gets to everyone (and then buff the acropolis some more). I'd also like to see cultural improvements give adjacency bonuses to the theater square, as that might be some interesting placement. I.e. a chateau or sphinx can give a standard +1 adjacency bonus, a great wall piece a minor one.
Chopping overflow "abuse" (debatable) results in crazy fast production.
===> Fix the overflow.
This seems straightforward - fix it so only the non-multiplied (and non-carryover) production portion overflows i.e.:
Finish a ship that had 60 prod left with 50 and a 100% card: nothing overflows
Finish a ship that had 40 prod left with 50 and a 100% card: 10 overflows
Coding-wise, it could just be adding a check on item completion to compare the base production with the amount that was left on the item.
Working district buildings needs to either provide double their current yield or provide an additional GPP per turn.
Double would be too strong imho for the first tier districts as it would led to people having say lots of just 'campus + library + specialist' cities (the opposite of boosting tall!). Wasn't it Civ 4 that ended up nerfing specialists out the gate because of a similar issue? From threads in the strategy forum here, the specialists can already contribute a lot more than people think (usually far outstripping the the adjacency bonuses for instance).
Adding them to the building output might be a good boost (so they get doubled by the policy cards when they come online) or having them tiered - i.e. 1st tier stays at +2, second tier +3, third tier +4, something like that.
My ideal world would be some sort of specialist 'adjacency' bonus where working specialists in a city gave boosts to other specialists in the city. I.e. working a tier 1 campus specialists gives say +.5 science output to all the other first tier specialists in the city if worked, etc. So it doesn't do much for smaller cities, but if you've got giant cities working tons of specialist slots, it ends up adding massively.