Different artstyle/animations something like in Prince of Suburbia, artstyle in SQ is not for me, ToP a bit better, HCH/ZR is on top.
But I definitely want to see changes of two things: main protagonists and more "end game" relationship/scenes.
Main protagonist in ZR 1/2 and ToP are short, 18yo, and their physique means that at most they can bring shopping from the store. HCH protagonist is a bit better and SQ, tbh I complytly don't remember him. Personally I really want to see a 'tall" (180cm +/- is enough), young (22-23yo, basically just after college), fit, muscular main character. Not a literal muscle mountain (don't jump from extreme to extreme) but you definitely can say that MC has no problem grabbing women ass, lifting her and fuck her brain out.
Harem relationship or however you want to call them should be in greater quantities and much earlier. Example: MC sit between two girls watching TV. One of girls get bored and decide to BJ him. Other girl with naughty smile seeing this move closer to MC but don't interrupt it. And here it is one girl suck MC while he gropes tit of other girl while waching TV, or MC being bored by film just pull cock from pants, girls look at each other and with naughty smile both of them start sucking him.
In short: different art style, different main protagonist, more "harem" interactions. Don't get me wrong, Siren makes great games but not the kind of games I'm looking for.