HardRocker said:
now I know that it's not possible for me to do it.
It's possible, it's just not simple. Honestly,
I think you should just try it anyway; make the change, and see what breaks when you try to play it.
The list of issues I posted can get you started, and others can probably spot more, but I'm sure you'll discover new problems as you go. We can help you design an intelligent solution to whatever balance issues come up, and you can think up solutions to a lot of issues on your own. That's actually a really good way to get started on modding in general; you get the playstyle you want in the end, and you learn how the underlying engine works in the process.
Nearly all of the issues I mentioned can be solved through either pure XML or by using several well-tested Lua functions. Most only require small tweaks, as long as you recognize that tweaks need to be made.
The few Policies that are now useless? Make some new effects for them that don't involve things like Settlers, roads, or trade routes, or changing the "number of cities" component in policy cost. Maybe a half-dozen Policies would NEED changing, and a few others would become underpowered without a change, but many mods have altered far more than that.
Wonders that boost things like trade routes? Change their effects to something else.
Too much room for Barbarians to spawn in? Change the default number of players and city-states for each map size, or give everyone bigger anti-Barbarian bonuses.
Civs would spend far too much time trying to produce a Wonder in their only city, only to see it fail and just get a refund, wasting dozens of productive turns? Lots of ways to tweak that, like changing Flavor values or giving the Palace a wonder production bonus. (Or remove Wonders entirely, if you want.)
Skewing the policy cost equation and Great Person thresholds? Change the constants in the equations.
City-states too strong? Lower the bonuses they get, bonuses that were designed to compensate for how large the major empires would normally be in the later game.
Conquest too easy? Raise the defense rating provided by a Palace (say, from +2 to +10), change the tech-related defense scaling, strengthen city ranged attacks, and/or increase the amount cities heal per turn. (I've done all of these at various times in my own mods.)
AI preferences still just way off? Flavor ratings aren't hard to adjust, although there'll be a lot of them.
No trade income? Add a simple +gold effect to some buildings that didn't have it before, or boost some of the ones that do. In the long run you could even use Lua to work out an actual foreign trade mechanism.
Resources too hard to get because of clustering? Change the distribution logic in AssignStartingPlots, like having resources be more clustered around starting locations (like how city-states work). Or, have some buildings create resources; building a Workshop could give 1 unit of Iron, building a Market could give a unit of Silver, while a Bank could give a unit of Gold.
India's trait is too awesome? You can change the numbers for TRAIT_POPULATION_GROWTH in the Traits table (it's one of the few that isn't hard-coded). Or give them a different Trait entirely, like just assigning India the unused "Philosophical" trait (which doubles your Great Person Point production).
And so on. It's not going to be a small list of changes, but it's nearly all XML-related. The AI won't handle this WELL, but it will no longer fall into so many little traps where it makes a choice based on normal game logic that would now fail miserably.
[If everybody thought the same things were "boring" and that "balance" should only be how they perceive it, there would be NO mods,
And again, that's not what I said. My point was that if you want that sort of game experience, you can do it, but if you take the easy path to that playstyle (just setting Cost = -1 and altering nothing else) then it'll break the AI in a large number of ways, because the AI isn't designed for anything beyond the vanilla Civ5 gameplay. You can get close to that playstyle (keeping empires small) through the relatively simple expedient of picking a small map size and adding extra enemy civs, to where each civ will only get a couple cities before they're out of room. But it'll still cause headaches in other areas, and if you don't fix these, then the AI will just fail to provide any challenge because it'll try to react to situations in the same way it always did, and those ways will now no longer work well. The AI in this game is just not very smart.