I figured I would assemble some of the more popular/ more discussed ideas here and in suggestions for 0ad.
Naval overhaul, including but not limited to:
scale down all ships
differentiate ships into different classes (transport, scout, warship, siege, special (e.g. fireship))
decrease dock build time
debate adding turret points for garrison
Unit specific upgrades:
0, 1 (town phase), or 2 (city phase) unit specific upgrades, see @BreakfastBurrito_007 post for examples.
available in barracks/stable, each unit type gets 2 upgrades, but civs vary in which/how many they have access to.
make siege towers a capture unit, with arrows a secondary feature.
additional statistic:
"effectiveness ratio" = value of enemy units, buildings killed/ resources spent
spec flare (only for specs, and options to disable/mute)
differentiate chariots, cataphract units.
Hyrcanian cav buff (2 options: stat increase and raiding unit)
https://code.wildfiregames.com/D4683 and https://code.wildfiregames.com/D4674 respectively
elephant archers increase rate of fire with each promotion, more XP to promote.
walls changes:
palisade resistance to cavalry damage
wall gate HP decrease, add a row of garrison space, doubling garrison points to 16.
make walls easier to place, perhaps destroying trees upon wall completion.
possibly a "fortified wall" upgrade available to some civs
CC cost/territory gain changes:
less territory gained in each phase, decrease CC cost
slightly less colony territory
raise all attack repeat times for performance, damage adjusted accordingly.
starting metal and stone:
not so close to CC
positioned separately on some random angle from CC with fixed radius
perhaps decrease to 3000, using 3 1000 res "clumps" of each metal and stone.