These ideas are not coherent. The reason you give for city sprawl being a problem is that you want the option to build "taller". So even though you might want to build taller you still want to be able to sprawl in other games, so it is not really the problem at all. It seems your problem is really that you want to build bigger, taller cities. Well the game does allow you to build cities quite a bit bigger than normal using the two spells that boost population growth. I think size 17 is practical in long games for your own race. But my main objection to such a suggestion is that this game is not about cities, it is about battling Great Mages.
I think you missed the point I was trying to make. That is there is NO option to build tall. I've used the population growth spells to get my cities to size 20 before but that's not really the same thing. All cities in the game grow at a fix rate that, aside from 1-2 spells, you have no control over their growth. So for example if it takes 50 turns for a city to reach size 8 then the person who builds/captures 4 extra cities will likely have twice the resources as someone who builds/captures 2 extra cities after those same 50 turns.
The obvious responses is often "Well sure they have twice as many cities why shouldn't they have twice as many resources." But this creates a very limited and uninteresting strategy setup. However if players have some form of control over how fast their cities can grow, other then casting a spell to make it faster, then this would add another layer of strategy to the game.
Like in Civilizations if you focus on food you can get larger cities faster sacrificing short term production. This leads to strategies such as culture victory where someone with a lot fewer cities can hold their own and win the game against someone who has claimed much larger sections of land. In warlock the only reason you can beat AI who control much larger sections of land is because the AI sucks so much.
cities are only a means to an end.
This reminds me of the mind set that seems to have taken over in recent years in the RTS market. With games like Dawn of War 2 and Command and Conquer 4, just to name a few, doing away with base management in favor of small groups of units and resource gains done through capture and hold style objectives.
There is nothing wrong with a new style of game that changes things up but they are no longer Real Time STRATEGY games. Instead they have become Real Time TACTICAL games which appeals to a different audience. The management of your resource income is just like the management of your Cities in Warlock are any other 4X game. It's a part of the overall strategy in the game, and having more Depth there will only help improve the game as a whole.
Please don't try to turn this game into another civilization, especially not Civ 5 with its disasterous mismatched economy.
JJ
As an Economics major I am well aware of the many flaws in Civilization 5's economy. And I'm not trying to turn Warlock into another Civilization's game. In fact just the opposite because I feel Warlock currently has several of the same flaws Civ 5 has.
The most obvious is actually the economy. In Civ 5 gold is king and pretty much everything else takes a back seat. I've tried lots of different strategies in Civ5 and so far I found the one that tends to work the easiest is to build almost nothing but Trade Post for extra gold. Doesn't matter which Victory condition your going for as you'll need LOTS of gold.
The same is true in Warlock as like 80% of your cities will be little more than gold farms. The lack of needing anything other than the bare minimum food for things such as Unit Upkeep and Population makes it a fairly worthless resource. I've even noticed you can have a large negative food income with very little impact on your empire. Mana is only useful up to a point because after a curtain point you can't use it fast enough due to casting limitation.
There is a problem with +10 pop growth per point of food - it doesn't limit the number of cities as if you have twice many cities you have twice the surplus food and thus twice as much to spread over twice as many cities.
Having the fixed bonus would work. Making in city food production more important would make things slower as cities need extra buildings for the food, so you would have to increase overall population growth so that cities have more buildings. It also doesn't work well with undead cities.
The +10 per food was just an example, balance testing for the correct amount would need to be done which would likely be much higher. It's also not meant to limit the number of cities you can have. It's meant to create a trade off of having either a few cities that grow quickly or a lot of cities that grow slowly. The choice of the number of cities to build is left up to the player as it should be.
The fixed bonus is to ensure a bare minimum growth rate since the starting capital would not have nearly enough food production to grow at an acceptable rate. As for someone having twice the number of cities and twice the food surplus that is a completely viable strategy as well. If they want to focus heavily on food production then that is a perfectly valid strategy, which is the point as right now there is no strategy.
Making food production more important would not make things slower, it would help reduce the importance of gold. Which right now the vast majority of cities are built for the sole purposes of producing gold. It could potential slow down city sprawl though if that's what you mean, mainly because people would then have to consider the fact that more cities means slower growth. However they could just build a lot more food production to offset this. The choice is left up to the player how they want to handle their city growth which is always a good thing.
I know that food wouldn't apply for undead. Since mana stockpiles unlike food a slightly different formula would be needed to apply to their cities. The premise is solid but all the exact details and balancing are things that would have to be worked out in testing. Since we can't make MODS we can't do the testing so that's up to the Devs assuming they like the mechanic's premise.
Conclusion
Over the years most 4X games have some kind of mechanic in place to curb growth and strength. If they do not then whoever has the larger empire ultimately wins because the other players have no hope of competing. This is done for balance since if a player is twice the size of another and thus is twice as strong. Thus the only way the smaller player can win is if the larger player is stupid. A system setup so that a player who is twice as larger is only 50% stronger makes things fairer as the larger player still has an advantage but the smaller player is not as nearly out matched.
This is at the heart of the "City Sprawl Problem". It's not a matter too many cities to manage, or clutter on the map, or etc. It's why in Civ4 they had increasing Gold cost per city and in Civ5 they have unhappiness increase per city. Different games have used different mechanics over the years and some work better than others. Like I preferred Civ4's setup over Civ5. When such mechanics are missing though it often leads to a city spam mentality and ultimately largest empire wins. I think this will only becoming more obvious when we are allowed to challenge competent opponents with the addition of MP.