Himeko Nova Kit theories
So, based on leaks and info and such, its largely assumed Himeko is going to be a part of an 'Astral Express' team. There are two ways in which this could work, number one is she will be like Cyrene, dolling out specific buffs to teammates if they are on a team.
https://www.reddit.com/r/HimekoMains/comments/1thhq99/himeko_sp_kit_theory_includes_leaks/
This post by WritingRiver, shows some ways the buffs could end up, while this would be fun, I don't feel it will be likely, as Nova Himeko is shaping up to be a main DPS/hypercarry, and this Cyrene sort of kit will likely end up with her in a somewhat weird place.
The second option, is the Astral Express team is more incidental. To see how we can look at the best members of the astral express, Welt, Sunday, DHPT, Evernight, and RMC. Of these, only Evernight is sort of left out, and based on these 4 and the leaked relic set, we can figure out some facets of her kit.
DHPT-This is the easiest, she'll scale off of attack, this is backed by the relic 2 piece giving her attack %.
Sunday-Also somewhat easy, with DHPT Sunday's value is better, and we can assume she'll work with a -1 comp. This is a bit counterintuitive, as from the relic it implies she has a skill like DHIL, which would be a skill point drain unless she does something weird with that. We also know she uses ults for damage though so Sunday's enery regen is helpful there.
RMC-She does damage with ults, and as a standard char, she should benefit from true damage buff fully, adds a FTP member to the team, etc.
Welt-This is, imo, the most interesting. Thus far, almost every novaflare has been made to sell a character in some way. Welt is an odd man out, as he wasn't the highest in demand for a buff, and doesn't really buff any new characters in a major way, or bring up a old team to usability again. This is further confusing by his dmg boost. A 10% boost per hit up to 10 times. This is somewhat weird, as not many characters can benefit from this fully, as 5 hits per turn is a somewhat large ask. With this coming when it has, I'm operating under the assumption that Himeko will work well with Welt.
Idea for kit-I'm not a game designer, so I'm more throwing ideas out there that sound fun to me, if we accept that Himeko is a skill/ult based DPS, who uses attack/crit/dmg for her damage, maybe she'll do something like 'when using skill enter a stance, whenever a team mate uses any action, she uses her skill, escalating to a 'higher skill level', from 1 through 3". This would explain how Sunday is her BIS, as she wouldn't be guzzling skill points, and he adds the fun synergy that he Advances her, she activates skill, DHPT dragon acts and triggers first skill immediately, etc. I assume her Ult is some massive damage button that needs a lot of energy, which means you want to get her skills off to battery it, etc.