Tinkering with pionner cost or gating them is not a good solution. Balancing mechanisms are sorely lacking in the game and must be added.
There are several simple solutions to be copied from civ, as well as other ideas:
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I like most (if not all) of those proposals. Very well thought!
- increase the benefit of large cities: tax, research, food & production becomes strongly (ie linearly or more) correlated to population size (kinda makes sense btw...), replace faction prestige by city prestige f(population, buildings) or give some buildings a prestige factor
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Isn't this already happening at least partially? A higher level city is much more important than several low level cities. If I remember correctly, gold/research production increases exponentially with each city level, so a level 5 city is like 32 level 1 cities.
- increase the inconvenience of sprawling empire: add a strong unrest component f(#cities), add a per city upkeep cost or city militia wages which scale with unrest but little or not with city size, add an unrest penalty to cities far from the capital (taking into account map size)
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The higher unrest depending on number of cities it's a good way to slow expansion. I disagree however with the distance penalty (unless it's distance to sovereign, not to capital). The city upkeep based on buildings and/or militia sound cool.
- add a min city requirement to produce settlers: civ requires pop 2, FE could require level 2 (that would be more a bit more obvious than pop level on FE)
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That would only slow initial expansion. Not sure if it's worth.
- on higher levels, AI starts with bonus settler(s): it's more fun to have them start out strong and struggle to catch up rather than perk them up so much that they runaway no matter what you do (ie on insane)
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Would be nice
- change outpost construct mechanism: almost any unit (heroes, human armies, unconsumed settlers but not caravans or animals) can initiate outpost construction, but actual construction is undertaken by closest city (just like for resources when they are within your territory) or by arcane monolith
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This one is a very good idea, and makes outposts consistent with the rest of resources AND with outpost upgrading itself.
- add a small military unit pop cost (eg 1 pop per squad member), add the same kind of small unit replenishment population cost (war costs population)
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This one is also very good idea. The game engine already has this scace resource called population, so why not use it? I would even go beyond and make unit replenishment cost all resources involved. Let's say that you have a unit costing 2 crystal 1 metal and 1 pop in a group of 3 for a total of 6 crystal 3 metal and 3 pop. If the group loses one unit in battle, healing that unit should automatically reduce that 2 crystal 1 metal and 1 pop from your pool. Make economy matter.