I think Slinger unit is good, like in AoE1. If the Slinger has a bonus vs cavalrys, it will be good, because I think now the Aztecs, Mayans (and Incas) don't have a "fast" or a 5 range anti-cavalry unit, like the other civilizations which have Camels (and Knights).
A video-audio recorder checkbox in the Options, which is when the game starts it will record the screen and sound of the game, like Fraps.
Balance: (just ideas)
Heavy Scorpions have +1 range (8 instead of 7)
Chemistry affects buildings too, like Town Center, Tower, Castle... So +1 attack for buildings too.
Maybe the Magyar Huszár is an upgraded version of Hussar (So in the stable: Hussar, next upgrade Magyar Huszár only for Magyars), like the Indian Camels: Heavy Camel -> Imperial Camel. But if this acepted, it will be a new Unique Unit for the Magyars, for example an another cavalry unit or a cavalry archer Unit...
Nice ideas i Think +1 and also the slinger to counter gunpowder units
Why only for the Incas?
Why not for all civs?
I think if they were the slingers to all civilizations, would go to the dark ages. but I still think it would be better just to the American civilizations
And about the portuguese civi i think that is possible if are obtained the graphics of the Unique Unit. Because they can have the same architecture of the italians