A well developed heroine would be nice. Not necessarily strong in the traditional way, but someone who you would like to meet in general. I think looking at visual novel heroines is a good idea personally, just to get an idea of what makes someone special, whether it be their strength or weaknesses. Consistency would be nice, growth too. Cardia from Code Realize for example made me go niya niya over her transformation and makes me want to back her every step of the way. Lulu from WOF was a bit ditzy but was willing to be bold and take initiative to make the story into a happy ending. I guess these are the two examples I really enjoyed in otome games (now visual novels hohohohohohoho)
It would also be nice if some of the story is told in the perspective of the hero whose route the heroine is in after their personalities are established. It would help to make them more real and I really like watching them fail and fumble a bit while figuring out their feelings or solutions. But that might just be my slightly sadistic side. Nah I just find it cute. Probably.
The personality difference thing for the heroine might be a bit difficult for a very complicated story. Examples of success include undertale and kakoware no paruma (I think that is how it is spelt), but it takes a lot of work writing and rewriting the scenario and organizing all the flags and branches. I suppose the easiest way for this would be to have a double heroine or triple heroine system, with two or three or maybe even 4 well established characters who have their own individual interactions. It might be difficult going along this route too but I guess shiro kuro alice kinda managed it? Lol I'm not too sure.
Good luck!