Prince de Radzivil said:
but I believe adding proper Frankish stuff (especially for cavalry)
Game is balanced for shield-wall instead of cavalry, as it is historical correct and also as a main difference between native and most mods. Not having heavy cavalry easy killing around may sound strange, it makes the game harder, but it also makes it unique, as much as the mods with firearms (relative to native).
Its really hard to cross the sea with horses. William the Conqueror did that two centuries later (in game era) and it was a huge factor for victory. It would break immersion if we had in 867 lots of horse fights. The alternative would be to program the franks AI to be defensive and never invade England (can do a raid, but not conquer territory, not send multiple armies with a marshal).
Or do something entirely different. In any way it would be a major addiction to the game that would require a total rethink about balance and game mechanics.
One major issue we would have right now: shield-wall in-game cant turn back rows. So if you have 100 spear + 40 heavy cavalry you would simple win every fight losing less than 5 guys. Just tank with spears, move cavalry behind and destroy them in one charge. They will die/rout in less than 10 sec. Rinse and repeat.
Another one: we would need a brace spear formation/command.
And so on.
TLDR: no addiction to the game is as simple as you may think when you think about history and game balance.