My proposals:
1. Variable aggression levels for each leader depending on era
I would prefer this over a general increas in aggression. This could assure, that e.g. Assyria makes optimal use of it's siege tower while the Ottomans will use their Janissaries.
This variable aggression increase might very well have a wide range. For example, Atilla might have an extremely high aggression level early in the game but only a medium level later on while Suleiman might have only a slight increase in the Renaissance.
2. Warmonger penalty depending on era
In early eras, war (or better: taking cities) is a diplomatic suicide. All civs have only a few cities built and the new warmonger evaluation kicks in heavily under this circumstances. The same applies for taking CSs as the smalest thinkable civs.
In reality, war was an expected and acceped behaviour in ancient times. Succesfull warring leaders were feared, but respected! "Morals and stuff" are more or less a modern approach.
I think, Civ5 should reflect this. Early warmongering should be way less penalized than warmongering in later eras. If at all, early wormongers should cause 'fear' in other nations and make unbalanced deals more likely to appease the aggresor.
3. A higer palace gold yield
As early gold (or the lack of it) seams to be one major reason for early peacefulness (see the 'sanity check'), why not increase the gold output of the palace? As the game progresses, this relatively small lump-sum of gold will make no big difference.
But very early in the game, the gold will help to maintain a reasonable army-size and lower the effect of trade routes on the mentioned 'sanity check'. Declaring war will therefore be more likely.