At risk of adding to this mega-necro
one of my aims in my own tweak-mod-in-progress was to de-populate the build-lists of inappropriate/ redundant/ obsolete units and buildings, especially in the late-game (Industrial era onwards).
I therefore have Cavs (+UUs) "Upgrade to: Light Tanks" at MotorTransport (A.D.M = 10.6.3, B.R.F = 8.0.1, 100s, needs Oil + Rubber; Radar, Blitz, Wheeled but ignores move-cost [=2] for Desert, Tundra and Floodplains*), but left the "Upgrade" unit-ability unchecked, to force the human player to build them from scratch, rather than stockpiling Cavs and then mass-upgrading**. If lacking one or both of the LT resource-requirements, Cavs can of course continue to be built.
I also find the one-dimensional "World-Domination by [Horse-unit]-Stack'o'Doom" strategy
really tedious (and being on the wrong end of it even more so!). So
all my Mounted/Wheeled/Tracked units have been nerfed relative to the stock-game:
— Maximum M=2 (UUs with M=3 got various stat-, terrain-movement, HP, and/or unit-ability buffs instead)
— Wheeled flag (if they didn't already have it), to keep them off unroaded high-defence terrain; exceptions for the "lightweight" Horse-UUs, (MWarriors, Keshiks, Ansars)
— On the grounds that mobile units should cost more per capita than foot-units, most M=2+ units get no bonus-HP, whereas "defensive"-flagged Foot-units all have +1HP from Muskets onwards, and +2HP from RepParts onwards (i.e. Mounted/Wheeled units have HP-nerfs relative*** to their era-equivalent foot-units)
— Default retreat-probabilities inverted, to make Regular units more likely to run away when redlined than Elite units
*
'Proper' Tanks must (also) still be built from scratch, and have been made more expensive (120s; Modern Armor now cost 140s), and require Iron (yes, 3 resources in total!). I removed their Blitz-ability (Panzer still have it, but now also only M=2) — but gave them defensive-bombardment (B.R.F=12.0.1), in addition to their ZoC. The LT currently uses the Tank's art-files, but my intention is eventually to direct the PRTO entry to use the "Scout Tank"(?) files from the WWII Pacific scenario.
**
Although the AI-Civs will still be able to upgrade Cavs —> LTs, that's not so much of an issue. Because once an AI-Civ has built a big Cav-stack, they tend to start the "Napoleonic Wars" of the pre-Industrial era — and once they learn RepParts, they tend to switch to building Infs or Guerillas as their preferred attack-units (= "WWI"!), because these defend better than Cavs. So by the time they learn MoterTransp, they generally won't have many Cavs left to upgrade...
***My basic "Ivory" [Strat Res]-requiring Elephant-unit — available to Carthage, Egypt, Zulus, India, Persia — has A.D.M=4.2.2 but gets an actual nerf of –1HP; WarEllies are A.D.M=5.2.2 and –1HP; all are Wheeled, but ignore move-costs of Marsh and Floodplains; and they can also build Roads and clear Forests/Jungle, faster than ordinary Workers