I think the biggest problem with keeping the existing girls as the main cast in a second game is the babies.
On the NTR path Ryan really doesn't have an option to pull out so it really just a matter of time before they get knocked up by their NTR counterparts.
Instead of keeping them as the main cast you could have the second game take place in the same town with a new cast of girls and a new MC. At the start of the game the player could be asked if they played the first game and if so did they go the NTR or Harem route.
If they pick the NTR the new MC would occasionally run into Jacky and the girls around town or at Joey's club. Maybe even fuck them if they are willing to pay.
On the Harem side you'd still run into them around town but Ryan would be with them and they'd be quite happy. Could be the player would catch them now and then fucking somewhere.
Further more you could set up the new cast in the first game. Ryan wouldn't get to do more than meet some of the new girls but it would establish them in the world so when the second game picks up the player would already know a little about them.
On the NTR path Ryan really doesn't have an option to pull out so it really just a matter of time before they get knocked up by their NTR counterparts.
Instead of keeping them as the main cast you could have the second game take place in the same town with a new cast of girls and a new MC. At the start of the game the player could be asked if they played the first game and if so did they go the NTR or Harem route.
If they pick the NTR the new MC would occasionally run into Jacky and the girls around town or at Joey's club. Maybe even fuck them if they are willing to pay.
On the Harem side you'd still run into them around town but Ryan would be with them and they'd be quite happy. Could be the player would catch them now and then fucking somewhere.
Further more you could set up the new cast in the first game. Ryan wouldn't get to do more than meet some of the new girls but it would establish them in the world so when the second game picks up the player would already know a little about them.