I'll just say this from a perspective of playing Honkai for a few anniversaries: Temper your expectations.
We'll likely get a log in event with wishes, a web event or two with books/ore/primos, a costume or two (one likely free, the other likely not), some minigames for small to medium amount of primos, and probably a selectable 4* character - likely out of a limited pool of Mondstadt/Liyue 4*s. I wouldn't expect them to give out anything like a free 5* or anything. Each gacha game has its own importance placed on anniversaries. FGO, GBF, and FFBE go all out for them, when other games acknowledge it and give a few bonuses on top of what they normally do. MHY seems to fall in the 2nd category.
I'd really like to see them change the "junk" items in the wish sytem to a mix of XP Writs/ore/Mora instead of worthless and cumbersome to fodder 3* weapons. That's my big hope.