About the power scaling of heroines:
Actually I don't know how balance them, but here are the things I will do:
1. Early stage heroines can bring more units (at least 60 hilichurls)
2. healing skills will be affect by element resonance just like samachurl's healrain
Furthermore, there is a plan to add recapture event for some heroines which will give a upgraded version of that heroine. For example: outfit Fischl, Hyacine Barbara, Herta Lisa and amputee Skirk.
Nice ! !
We could have a Knight Noelle ^^
And the idea of recapture... I like it ^^
The healing scalling will be really nice ! !
The extra troop will also be nice.
For the balance, I could think that they could also gain raw stats/elemental affinity, with it being capped by your resonnance, and requirering a long process (like some time at the altar to pump sor abyss energy, or at some other place). If the place has limited spot, it would prevent from just upgrading everyone, and the time would put it in competition with other things, such as breeding or ressources production if it's the altar.
Another way I could see it would be to need a higher power heroine of the same element to gradually make them catch up the difference, but it require to dedicate both heroines during the time needed (with a long gradual process that can be interrupted at any time, potentially drainig both Resistance). Once again, it would be held back by the compatition with breeding (especially if it need a stronger heroine in addition to the upgraded one) and the limited availability of heroine strong enough. I would also naturally cap the point to which they can be upgraded.
There could also be be farmable consumable to give heroines to increase their power. It could mean obtainable via the hard content (but with a capped number of use per heroines), or litterally farmable (but the farming spot is either unique/very limited, like doable only in certain settlement of with a specific material, expensive to build/keep active, progression/capture locked or ververal of theses at once). I could imaging thing that gives a third action, or elemental bonus, for example. The capped number of use would prevent if from getting too out of hand (but late game, such content can be clears 1-2 times/turn for chasme, and once every few turn for Musk Reef), and the farming would put time as a limiting factor.
They could also recieve a bonus on their skill an burst based on the stats of their troop, like the HP/Atk/Def scaling of the base game. I would naturally make them scale with stronger (and well chosen) troop and troop size upgrade.
Finally one of the things that weaken early heroines if their limits in the troop thay can bring. A way to increase that would be quite helpfull (it could be part of the recapture upgrade, require a specific consumable for each unit to unlock, of a specific objective for the heroines (like making Noelle a knight, capturing Jean for Barbara, giving Xianyan a stage for her representation...).
It seems that game-mechanical upgrades in Genshin Impact is something only the Traveler can do. But here is the thing - in Hilichurls' Ambition, we get to capture the Traveler (Lumine), so she can teach us to do it. Alternatively, a Katheryne can do it.
Or we can just give them better weapons. This seems to be available for anyone.
Great ideas !