Castles an walls should obsolete with cannon, since thats the unit that effectively allowed attackers to take heavily fortified cities that in the past would have been very difficult to take much more easily.
Castles should be available with construction, were they provide an additional 50% for 100% total, with the discovery of engineering this should increase by another 25% for a total of 125% def, big castles of the day were some of the most difficult structures to lanch an assault against, an example being the castles built along the coast during the crusades, these had stockpiles of food an supplies to withstand a siege of 5 years, i do not feel castles are given a true representation in the game in terms of there defensive attributes there more like walls mark II which is simply not the case, the above would also solve the complaints that they obsolete to early an are not worth getting.
The explorer unit would no longer be available (it's totally pointless everythings already been discovered) in it's place would be a recon party with a site radius of 2, making it a useful unit for giving your stack a greater site radius an keeping an eye on things, this would later upgrade to a modern equivalent Army Rangers or somthing like that with a 3 site range and the medic promotion.
Marine an paratrooper would merge into a later modern era commando style unit, with the robotic's teck, theres a thread on that already open, since mech inf pretty much obsoletes marines outright, an paratroopers are to niche in the late modern era.
Somthing along the lines of
str 24 + 2 FS
starts with commando, amphib, paradrops
Longer range paradrop than current paratroop, can paradrop from aircraft carrier, aircraft carrier can carry 3 of them
only takes 2 turns to fully fortify, + fortify bonus is 50%
Naval combat to be given a major overhaul so units are structured in a style that promotes variety, take land units, axes counter swords, chariots counter axes etc, the naval works fine up until destoyers (mainly because the older sailing ships each have a more specific role an function meaning they all have there uses), when it seems to revert to a destroyer + the odd battleship navy, the other naval units (subs really) have combat power so low that they cant sink anything except a lone transport, destroyers should have a major bonus vs subs, subs should have a major bonus on the attack when attacking surface ships, they should all be worth getting at the moment destroyers with the odd battleship rules, subs are a waste since they are outclassed by everything bar non combat ships, an since the sub is waste that means it's counter the attack sub also has little function, considering the importance subs play an have played in naval warfare it's shame there so lame toothed in the game
Something along the lines of
destroyer str 30 + 75% bonus vs subs
Battleship str 36 + 50% vs destroyer
Sub str 24 with 1 first strikes + 75% attacking surface ships (can carry 6 cruise missiles)
Attack sub str 24 with 1 FS + 75% attacking suface ships, + 50% vs subs, has 1 more movement point over a standard sub
Stealth Destroyer as destroyer but stealth system can be detected by all units that can spot subs, SD can spot subs (it cant at the moment)
Missile cruiser str 36 + 50% vs destroyers can carry 8 cruise missiles
Naval promotions with a bit more strategy to them like the land units get, sonar upgrade + 25% vs subs, formation upgrade for ships, + 25% def on the open sea, High explosive topedeo's subs + 25 vs all targets you know what i mean stuff along those lines rather than simple combat stars an drills
Promotion tidy up, gunships can still access the shock promotion, why? it's totally pointless to have that on a gunship, while it looses the siege promotion, that was available to cavalry, so the promotion thats still relevant an useful you can no longer get one you upgrade to gunships while the one thats totaly outdated you can, it just looks ridiculous, mech infs can get the cover promotion? again why this promotion should obsolete with mech inf an no longer be available since archery units are long gone by then.
Airships should have a counter unit, available shortly afterwards a ww1 style plane with a 40% odd intercept chance rate or some such so your not competely defenseless but at the same time not so good that it makes airships no longer worth getting.
Those are my main gripes to be honest, or at least the ones that spring frequently to mind
