1) Increase food/strategic resource yields, increase production yields in mines with tech (already done in mods)
2) Reduce TPs to 1 gold (already done in mods)
3) Make CSs give you a percentage bonus to your own culture/food production in a limited way rather than gift you stuff for free.
So that a maritime CS gives you % bonus to food production in cities, rounded down.
And cultural CS gives you some small % bonus to culture production in cities (NOT TO THE TOTAL), again, rounded down.
(not to the total, so that you'd have to specialize some cities to be culture spots, which again refuses ICS viability)
That way you have to again specialize your cities and give them unique strategies.
Also, you don't get a ton of free food for nothing and your ICSs cant grow without producing REAL food (the core of the ICS strat)
Also, you don't get magic culture, but again, have to produce real.
Also, production becomes a bigger focus than purchasing (Because of the tile/improvement yield changes), which in turn promotes the creation of bigger, better placed cities.
This setup still promotes expansion, as a CIV game should, but more thoughtful and planned expansion now.
ICS fixed.
1) Increasing food yield will still leave large cities very hard to make. The problem is they grow way too slowly and there's not much of an advantage to having one big city compared to 3 or so smaller ones (culture is just about the only thing).
2) Normal Empires can already have significant money issues. Now you've just bankrupted them and made non-river starts such a joke that no one should ever play a game without a river. Nerfing TPs is a gross overreaction. They aren't the problem. The problem is everything else. Something can look overpowered when the truth is everything else is just too weak.
3) You don't need Maritime to have ICS work. You only need size 4 cities. Even anything Maritime is a bit problematic in Sulla's game because it made his cities grow too much (giving him extra problems with happiness). This makes the initial period a little slow for a new city (you use farms to get to size 4, and it makes really bad areas like ice and resourceless tundra not worth it..probably). Maybe toss in a Granary. Besides a small delay for new cities getting up to snuff, it doesn't change anything.
I don't think these fixes would work well together either. Now you have big cities that can make stuff reasonably (assuming we are doubling hammers here), but you have far, far less gold to support more buildings OR more military units. How does that help anything? That would tend to encourage ICS anyhow (with more hammers meaning a small city doesn't need communism, necessarily).
With any of these things, you need to consider why building more cities is bad. In Civ 3 it was corruption (which was insane and awful). In Civ IV it was city maintenance, something a small city couldn't avoid. In Civ 2, of course, there was nothing, despite bigger cities having more production and being able to produce more -- kind of like what you are proposing here.
Personally you need some sort of cost for each city. Happiness isn't a very good way to go, imho, because while it might work, it doesn't make much sense at all (and it would probably hurt more reasonable empires). Some fixes probably need to be done with happiness though (I could see putting a pop requirement on Theatres and Stadiums, then making them MORE efficient than Coliseums).
So what else do you have? Culture? Already being tried and it doesn't work. What you have left is science and money, pretty much. Well, I don't think a science hit is going to do much, personally (worst case you fix it with some big cities or again it becomes an extremely artificial mechanic). Money seems like the best bet. City Maintenance made sense in 4, didn't feel like an artificial mechanic designed to punish you, and it worked at stopping ICS. Naturally building maintenance would have to pretty much disappear, but again that also is anti-ICS since it means big cities don't have crazy costs that they have to pay compared to small cities.
Of course, other stuff needs to be be adjusted to fix the other problems Sulla's game revealed (such as terrain not mattering, etc). I can't think of a better and more fun mechanic to help fix ICS than city maintenance, however.
(and did i see a horse somehwere 
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