I think inventions should basically occur as modifiers on an object that are influenced by necessity, should be a very slow thing to occur, and should occur to all civilizations randomly, and will replace the default item if it's better. Some examples:
This is a stone mechanisms. This object has been miniaturized by researcher (insert name here), who (insert story relating to some failed production order here). This object costs one half of the material compared to its predecessor. This object weighs one half as much as its predecessor.
Using a mechanics workshop would make mechanisms in batches of 2.
Maybe some special properties:
This is a lever. This building has been enhanced by researcher (insert name here), who (insert story relating to a fortress destroyed via magma flood here). This building activates on contact with water. This building activates on contact with magma.
How many fortresses could that save?
As for weapons:
This is a steel bow. This object has been enhanced by researcher (insert name here), who (insert story relating to the elf/dwarf conflict here). This weapon automatically loads. This weapon automatically fires. This weapon has a fire rate 2 fold of its predecessor, which in turn had a 3 fold fire rate improvement over its predecessor, for a total of a 6 fold fire rate improvement over the original design. This weapon has an additional cost of 2 mechanisms to produce. This weapon is produced at one half of the rate of the original.
Imagine an elf carrying that thing.....
Although, once the army arc and all that stuff is implimented, there would be issues with more advanced nations stomping on the less advanced nations... which happened in real life anyway.
[ July 12, 2007: Message edited by: AlanL ]