Aloha,
mircea74 Human player conquers enemies too soon and most of us never have the chance to use advanced late war equipment. And it's not that challenging after 2-3 years. I know i felt like it was a downhill after 2-3 years
Well, there is some truth to that but personally I dont see much ways to change this, perhaps increasing corruption (in relation to distance) will lessen the effects of having a vast empire on science and on the treasury thus levelling the playing field some.
mircea74 Technologies should cost less. I was always behind at least 1 year in the technology that was supposed to research
The reverse is more often true in my games
mircea74 More upgrade paths
Perhaps, but Id actually remove the upgradeability of the Pz III F/G to the Pz IV H because at the point of researching Land 1943 one suddenly has hundreds of lethal tank units at ones disposal ( The upgrade path of the Me 109 is really spot on very well done!)
mircea74 Tiger II for example should have -1 HP and a very high price to reflect their reduced usage.
The Tiger II already has an unfavourable cost vs. value relation and the main reason I do build them in great numbers is to limit the number of my army so I dont loose track of all the units at the late war stage.
mircea74 I don't like when i see so many Hindenburgs, BBs, CVs that were never in use by a specific country. Make some small wonder instead with auto-produced units
Personally I could live without the Hindenburg, primarily due to Germany already having the best units in most areas.
An auto-production option for capital ships would also help to put more emphasis on the cruisers, Naval bombers and especially the subs.
mircea74 Also HP's on U-Boats and all subs should be reduced IMO. Large attack value and stealth should be enough for them. Same for Marines, Paratroopers
Subs with fewer HPs had the advantage of simulating the thread of being attacked by air much better but then again fewer HPs would dramatically reduce their effectiveness as a weapon of war. Up to now a good sub can sink anything up to a Heavy Cruiser on its own while being rather vulnerable to enemy attacks and unable to inflict great damage on large convoys as a whole.
Paratroopers were elite units (Mt. Cassino, Bastogne and such) they have to be of use in regular conditions as well.
Other ideas:
Some of the units can not be disbanded
which forces me to send them in some kind of suicide attacks in order to reduce my military expenses.
No arty units for major powers, they in return get some siege arty (rail guns or such monsters such as Thor and Loki

)via wonders with long production cycles.
Oh while I am at it, long live the Me 410 and He 219! Is there a reason to build them?

I just feel so bad for not building them despite their beauty
And some great praise of the Fortresses and their effects on game-play (impetus to bypass strong fortifications and simulating heavy casualty city fighting)
The issue of trans continental trade (destruction of trade wonders) has already been raised before but some ideas about it. What about repositioning the trade wonders more to the central locations of the continent in question, in order to let the defender enjoy the advantages of trade longer and to better simulate the initial supply problems of the invader (from London to the Middlands, from DC to Denver etc.)
All the best
Baldurslayer