You just have to look at their logo to understand this, because their logo clearly convey their vision on games.
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For Illusion, their vision is
Impression,
Inspiration, and
Innovation. The
Impression is the key here. They created a games that they think will have a deep impression for their customer. They don't shy from unconventional and sometimes controversial takes. That's why they have many girls with truly diverse personality ranging from meek to straight up psycho. From a cutie, innocent girl to manipulative, sadist, masochist, and even coldblooded murderer. From a girly, to tomboy, to gals.
That's why Jinkou Gakuen/Artificial Academy is such a masterpiece, because they have so many diverse personality to play and interact with. It shows in their spoken words, and it shows in their action. Koikatsu is a more "tamed" version of AA, with more cutie, lovery-dovey tones rather than more realistic, harsh reality of Artificial Academy. And
every games of Illgames is
MUCH MORE toned down version Koikatsu series.
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For Illgames, their vision is
Innovative,
Liberty, and
Lovely. Take a notice on the word
Lovely here, they wrote "spreading loveliness (charm that makes people love you)". So what they wrote here essentially is they wanted to make a "lovey-dovey" games only. Something that is not bound to uneasiness.
They would shy away from anything controversial. No harshness in the spoken word. No threatening. No sadistic/masochist/psycho behavior. No manipulating words. No physical violence or murder. Just lovey-dovey, sugar-coated interaction, which, ultimately, as you said yourself, lead to "blandness" personality of all the girls in every Illgames games.
That's their approach to games now, a 180 degrees flip from what makes Illusion games so interesting. They may based the decision after reflecting the "controversy" Illusion product has stirred in the past, and they don't want anymore of it. Or maybe the bishoujo games landscapes is changing and lovey-dovey interaction is what their customer in Japan really want right now.
They still can go back to their former self if they want, but I really doubt it lif we look at how succesfull their games so far. We shall see....