I have been playing this game for a while, and it's on my list of potentially awesome games. The dialogue is a bit stilted or forced at times, and the spelling gets wonky; but I can live with that. Especially as, some have told me, the developers might not have english as a first language.
With that in mind though, there's a couple things wrong with the game. Wrong as in, they could have been done better, or maybe it's something to consider.
1.) The girls shouldn't be 'easy' per se, but there should be some kind of option at least early on, to get around the IMVOIS/VOIS with one or more of them. It doesn't have to be sex, per se, but maybe a chance through various dialogue options to put the fact that he's a mediwizard to use. Something like that would keep player's attention, but it doesn't have to be "hey, let's fuck".
2.) Even with hints, the game is very vague on key points. Some of that vagueness is understandable, and warranted, but most of it just makes things very confusing and frustrating. For example, consider if you will the catalogue needed to progress in act/scene 5. There's no real instruction on how the player goes about getting it, and if you skip to that point, good luck in figuring out what you're supposed to (or likely should already have) know to get it. There are other little eccentricities of the game that play into this. Such as having to talk with the painting, then back to the hat, then try to re-read everything and figure out what the girls want... That probably could be simplified some, but on a first blind playthrough, it's very vague as to how you should progress.
3.) He's not a very good wizard is he? Realistically speaking, you'd expect that he would know a great deal about magic, yet the game seems to suggest that not only does he not, what he does know he's very bad at. It would make things more playable if there was some interaction ability when it comes to magic. Perhaps making it so that the player has another stat they can study, which unlocks (over time) various useful, and fun spells. For example, he studies and gets a one time use of a spell that makes a girl flash him, or hike her skirt, or something. As the spell is one time use, to re use it the player then has to study the specific spell. That or put it on a long cool down. Either works. By breaking away from the existing stats, you open up the possibility for new play through. Imagine if you will, using a curse on Daphne that makes her come to his next meeting without any bra on, or panties, or both. Alternatively, imagine a spell that makes hermione suddenly cum while talking with him. Naturally this adds to corruption of the girls in question, but also could add some fun options for the player.
4.) The animation is good, but it seems (at least at a glance) that outside of Susan, all the girls have pretty much the same sized breasts. Variety is the spice of life. They don't all have to be massive, nor should they be flat chested, but the ability to at least see some visible difference would help.
5.) Remember where I said frustrating and vague? Well, in all honesty, sonya really doesn't help. So more functionality there would be great. Something that gives us a reason to bother with her, other than just eye candy.
6.) More options and places to go. Even once you get the ability to get out of the office, you're still limited to only a handful of places. It would be nice and could prove interesting if you could go to Hogsmeade, possibly running into one or more of the girls there. This could add a story point, where the codex rules don't apply, since they're not on the school grounds. So, if your waifu is happy enough, maybe you get to sneak off the grounds and have a bit of fun.
7.) Couple of quality of life things to consider: Having to click everything that you've unlocked in sonya, could be simplified as it's already checked. Unbind skip from a keyboard key. When testing different girls, it got annoying that the skip seemed to work hit and miss, and I had to reach to the keyboard to do it. Most vn's have it where you simply tick "skip" and it does it automatically. Minor bug fixes, and rewrite of text for grammar and spelling errors.