Promotions
1. Abandon promotions that are only or at least much more benefiting the human like blitz (so absurdly imbalanced for ranged units/ships and the AI never gets it), +1 range and march.
2. Another big problem is the ground based system of the promotions. The human player is clever enough to kill the AI units with 2-3 heavy ground promotions in the open, where they effectively have no promotion at all. So I would suggest to go back to civ4 promotion system (if possible). Instead of ground based bonus (open/heavy ground) give +10 % combat strength per promotion. This would benefit AI and human alike and not only the human.
Units
1. Weaken all ranged units ranged damage severly (perhaps halved), but increase their melee a bit. So an adequate part of the damage has to come from melee units, which would lead to losing more units. And no range 3-unit before rocket artillery.
3. Ships before Destroyer/Battleship should have only melee range. The AI has absolutly no idea how to use a fleet (if it feels to build one), so this would give the AI a chance to defend coastal areas. And wooden ships firing up to 3 fields inland is only annoying from a realistic point of view. (Should be possible with the addon.)
Promos
1) Have to agree with Blitz; alternatively just grant it (the +1 attack version, of course) to tanks as a native promotion and make it CannotBeChosen. +1 Range is what I always rush my sea units towards, granting them huge advantage vs. other shipping and cities.
What to do with March?? It's granted to all units by the Pentagon, which I WILL get if at all possible. Also, March grants healing to naval units without Recon2 even in enemy territory (or Recon1 in neutral).
Addendum: Evil thought -- Grant all Barbarians March promotion. /Addendum
Do you use the Medic promotion? I have NEVER seen the AI have it (not that the AI would know how to use it).
Yet another unfair player advantage.
I have taken to have Medic require Shock/Drill_3 instead of Trenches/Guerrilla_1.*
2) While the AI concentrate all their non-horse/armor/siege promos on heavy terrain, it leaves them wide open on the flats, where I put half or more of my promotions. I have tried altering open terrain OrderPriority but it has had no affect.
What DOES work to a quite satisfactory extent is: Drill_1 requires Shock_1 and Guerrilla_1 requires Trenches_1; reduce requirements of promos requiring Shock/Guerrilla_3 to ... _2 as desired. The AI mixes some of their promotions among open & heavy, and you also see some strong open-terrain units.
Strong player advantage essentially neutralized.
Another problem I have with Thal's promotion philosophy is the strengthening of so many promotions; e.g., adding +1 sight or movement to several vanguard & naval promos, in addition to anti-fortification/city promos becoming so powerful that being fortified or a city can be a BAD thing! I find it 'wrong' that a frigate has such sight-range that a fighter on a carrier is no better.
I intend to reduce Drill, Barrage, Siege & Volley strengths by 10-15%, but add +5% vs. fortified to Drill & Barrage (problems of rough, fortifications & cities are similar).
Units
1) Could work (re-envision the siege unit as siege-supported melee), but a player will still (if necessary) wait until there's only 1HP left before finalizing the assault. City assaults should not be lossless, regardless of city condition.
The effectiveness of an area-effect weapon should decrease as the target density (i.e., hit points) decreases so it would be bleepin' DIFFICULT to destroy a unit with ranged units!
3) I have toyed with the naval 1-hex range, and in conjunction with reduced sight-range I just MIGHT do so. Surprises (even disasters) are a GOOD thing!
(And I play on LARGE maps (epic speed), but at Prince difficulty).
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* My alterations to VEM suspended during the Beta versions
I remind those who disagree with my philosophy to read my sig. I prefer immersion to fun, I play much of my games in role playing mode, and I suffer from Rommel Syndrome.