First of all related to the initial features:
2. Related to futanari characters at this point I would tell you to just focus on them and leave out male characters, since they are a significant rol there is no reason to not just be the main one by having a world in which the MC accidentaly appears and that only has futanaries and females.
7. It's fine that the girlfriend is like that but I would think in a game like this the risk to fall should exist.
Related to what you are looking for I'm just gonna share a general idea:
-Story: The MC and his girlfriend were having a date outside their village when they found a shrine that they didn't know it existed, out of curiosity they decided to enter and found that it was dedicated to a deity that had a female body and a dick, exploring the shrine they found a few weird items and a message, after finding out it's meaning suddenly they got teleported to an unknown forest.
At this point the couple will try to explore the area only to find a few futanari goblins suddenly ambushing them, while they tried to fight off against the goblins they just couldn't and were defeated, but at this point a futanari adventure of the nearby town that was doing a quest to hunt these goblins appeared and rescued them.
Notes: Contrary to many games, their rescuer isn't someone that thinks with their dick but an actual overall good person, that doesn't mean that she can't end up being interested or falling in love with any of them.
-Engine: I would recommend anything that helps to make a RPG game.
-Gameplay mechanics:
*RPG focusing on dungeon exploration, town development and house development. Join the guild, take a mentor to teach you how to fight and do quests to earn money for daily life or some luxuries.
*Discover relics, skills and secrets in unexplored places that will help you in your adventure.
*Find the omniscient crystall ball, a treasure that will help you receive a warning about hidden plots and let you see where your girlfriend or other characters are.
*Love interests and rivals: I would recommend having 2 main rivals and 4 additional main girls other than the girlfriend.
*Not only focus on battles but also on sex stats: The MC can practise his skills by paying a prostitute in town, by paying her the MC can improve his sex stats from E to S level, the effects being:
a) Friendly talk: The higher the level the easier will be to improve the relationship with a character from no relationship to best friends.
b) Seducing talk: The higher the level the easier it is to cultivate love feelings with a character.
c) Sex skill: A very important but costly skill to improve, have many practical uses including obtaining more relationship points with a character after sex, slowing down NTR falls through sex and even being great to make a fallen character fall in love with you again.
*Money is a resource, pay taxes, rent and daily necessities, failing to do so will have consequences.
*If you end up in a bad situation by paying a really high ammount of money as an offering to the goddess you can reset the world relationships to the initial ones (basically soft game reset of relationships).
Main rivals:
a) The seducer: She is a futanari girl that already has 2 girls as her harem (not main girls but side cast, side cast in the sense that they can only be with the seducer or with the MC being much more limited than main girls), she has fully trained them into sluts that exist to please her and is looking to have fun, overall she will try to push the MC into what she calls "sex battle", this would consist on the MC choosing one of her girls to seduce while she will do the same to the MC's girlfriend, wins the one that makes their objective fall for them first.
Since it is extremely unfair the MC will show no interest in it, however she will send her girls to tempt you, for how long will you be able to resist before you fall into her game? and will you be ready to actually win it?.
Overall this rival is more playful than anything but will earn a huge interest into the MC if he manages to win the game, making him the target.
The situation would evolve like this:
1º Interested into MC women, will push you towards sex battles.
2º Interested into the MC, she will actively try to seduce you and if successful she will also take over your harem of girls.
3º Willing to be part of the MC harem.
b) The harem seeker: An easy going futanari adventurer in town that simply wants to build up a harem for herself, contrary to the seducer that only targets your girlfriend/harem members, she will go for any girl that earns her interest, always pushing boundaries with girls however if you are her best friend she won't target any girl that is your girlfriend or that is in your harem.
Overall if you manage for her to fall in love with the MC she will start to actively seduce the MC to the point that she would even give up her harem if the MC becomes her boyfriend, if the MC becomes the bottom in said relationship she will make the MC be her exclusive partner and will enter a monogamous relationship with each other.
Situational rivals (basically rivals that could appear after achieving certain conditions):
a) The infiltrator: A futanari girl that is great at illusion magic, she will seduce the MC appearing as a girl and join his harem with the only objective to take over his harem from within. Overall you just need to discover her hidden plot to stop it, which would result in her capture and you building a dungeon in your home to train her as punishment. Depending on how much she advanced her plot she will either be directly captured, escape with the seduced girls out of the town and having you go to try to capture her or she will take over your harem and enjoy teasing the MC about it.
Conditions:
-Have a harem of at least 3 members
-Have a big house in which you live together with them.
b) The cursed: The futanari adventurer that rescued you both at the start got cursed in a dungeon exploration, now you need to rescue her back, go with your girlfriend to the dungeon in which she disappeared and find her.
Under the curse she will try to get your girlfriend and fuck her, if she manages to fuck her enough at the end of the dungeon when you solve the curse they will enter into a relationship, however she will feel really bad about taking your girlfriend and will offer to let you see them have sex
If not she will fall in love with the MC and she will declare her love for the MC, at which the girlfriend will say that she is starting to regret the rescue.
Conditions:
-Your girlfriend hasn't been stolen by another character
-Enough game time has passed.
Overall these are just a few examples to go outside the rule and give some interesting twists. Situations either can make the MC recover from them or can be reset by paying a huge ammount of money, also you can just enjoy situation. Overall the concept is more about MC trying to make a harem or at least protect his current girlfriend and he can actually succeed and be happy.
