The most popular adult games have all mc's with big dongs, the most popular futa artists draws futa's with large dongs 30 cm+ etc. they may not make them a dong on legs in their serious works but they draw them, and even those dong on 2 legs have a big fan base.
In the first place there isn't a game made with Koikatsu where the mc is a futa with a 30 cm+ dong the one that comes closest to it might be Futa concotion but the author just plagiarsed some official works (hero academy, one piece etc.), and didn't put enough effort into writing it as of now atleast.
The most popular Koikatsu created game is Twisted World the mc has a large dong 30 cm+ and the author snats has 890+ patrons not to mention the other support sites and that with his lowest donation being 2 dollar speaks for itself, and even that without talking about that all of the limited high pay options are "sold out".
Taffy Tales has a mc with large dong 30 cm+ with enourmous user base while many left because they put more and more netoare elements into it ignoring the fact that the games success is based on the beautiful artworks and far less in contrast to art but good written adult scenes.
Fantasy Opposite one other game on the rise of the games created with Koikatsu has a mc with a larger dong than any of the ones I currently listed the only reason it isn't on place 2 is because the author is lacking writing skills and the choice to make the mc so pathetic and subby.
Summertime Saga while the mc has not a dong as big as the mentioned before is also a game with huge user base, the mc also has a big dong dunno how big maybe 26 cm+?
So to correct you the current most popular adult games all have a mc with a big dong, that is not to say this author should do exactly the same but I know no futa game that managed gain even a 10th of the popularity of these games when the futa has a bit bigger than average dong, honestly I think going with a the chastity belt suppressed it and it grows suddenly larger wouldn't be a bad idea, it naturally doesn't have to be a dong on 2 legs level but atleast so much that you could say mc has biggest dong in this game.
Especially since there is NTR in this games name which will prevent it most likely from gathering a huge user base so having the safety of mc has biggest dong may prevent it to be a hindrance, because most people don't like Netorare, I still think the author should change it to Futas World of Netori to lure a bigger user base and not scare people away.
Also to note is that games with transformation are really popular many text based games gathered a Huge user base with giving the user the ability to create a character as they wish (for Example Corruption of Champions 2, Trials in tainted space and Degrees of Lewditiy.).
Honestly i'm actually just writing this 95% for this games creator since I don't want to see a talented content creator stop because of lacking userbase interest.