Guide on adding a new model, animation, and creature to HoMMV

So, what are the chances now? I may have a negative attitude towards the third game (I'm a fan of the fourth, but I respect the third for its AI), but it simply has no chances when compared to the fifth. Comparing the third and fifth games, the fifth is just a clone of the third, but in 3D, and not the most successful one in all aspects:
1) Let's start with the AI, which is relevant today. What prevented the developers from simply taking the AI from the third game and not reinventing the wheel for the next four games (this applies not only to the fifth).
2) I won't compare the graphics of the third and fifth games; just note that there were games with better graphics than the third, even before the fifth, and without all sorts of unjustified bugs and functions that don't work together or don't work at all.
3) Gameplay and balance: here, things are the most normal. Necromancers dominate everywhere and always in the late game (almost). All that's missing is an artifact to summon liches for 0.1 units of energy.
4) Modding: this is the most interesting part, specifically the fact that it is very limited by the ingenious decisions of the developers in the style of "we are the developers, so only we can do this." Listing everything that cannot be done because of them or can be done through workarounds (hex editors and scripts) would take too long.
In my opinion, the third game is still popular because its source code is available online, and as for the code of the fifth game, everything is clear.