Aztec : just add a chinampa (see topic) as replacement for the fish trap. Maya can be splitted. New architectural set for the aztec. New civilization (tarascan, purƩpƩcha). New regional unit.
For steppe civilizations (mongols, huns, cumans, khitans, not the tattar a poll say that they must keep the farm) add pasture (and balance the pasture) Also new architectural set (western and eastern steppe), steppe camel as regional replacement for the camel rider (for the tatars too)
Britons + Celt
A new DLC splitting those two civilization britton will became the English (they keep the actual britton icon) and gain a footed mele UU. Longbow become a regional replacement for crossbow and arbalester for the english and the welsh (new civilization)
Celt become the scott, replace the woad raider by the gallowglass.Woad raider UU are kept for a pictish civilization or moving to a chronicle civilization. Irish will be added.
Also adding a saxon civilization featuring the anglosaxon will be good with huskarl as a regional unit for the saxon, viking, goth (and upcoming civilizations : vandal, suebi)
Japanese : add the tanegashima ashigaru as a UU who replace the handcanonner, a weakest and cheaper version of this unit. Also we can split the japanese in various faction with a sengoku DLC or add the Ainu.
Goths need a rework to remove their gunpowder unit. Maybe as part of a migration period DLC (see topic) with suebi, alans and vandal. Huskarl become a regional unit.
Spanish : renamed castillan and new civilization will be added : basque (Navare kingdom), catalan (Aragon kingdom). Caravel can become a regional unit.
Byzantine doesnāt nedd a rework, only a new architectural set with the bulgarians, georgians and armenian.