Kventin
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2. Regarding diplomacy: I would still add the option to join exclusively for money, but at twice the nominal price and only for units of levels 1-5 (following the principle of the reduced hill fort in Heroes).
We'll see. From what I've tested, there's definitely no overpowered strategy on JC: up to level 121, 1-3 joins, with only one being a strong one.
You need to quickly level up your hero, accumulate an army, possibly money, choose, and reach the necessary neutral creatures and the main goal, while still attacking the center.
The overall buff is comparable to maxed-out luck. The pace certainly speeds up, especially with the Tower. But it's not that significant compared to the Moon.
Besides, a main character focused on diplomacy might not be the best solution against a warrior. And if they are providing support, the benefit from them will be less.
On other templates, there may of course be some nuances.
It's not a problem to adjust later, based on the identified facts.