I love the game.
Somethings that hopefully will be added is a way to lower a partners tech. You can increase it but it should be a way to decrease it as well.
Also a good idea is to probably either increase the cost in materials for like the drills and special items since after a while you will end up with unlimited stuff.
If you are adding in more characters you should have it so that you have to build the extra space to bring them with you. This gives useage to the materials. Maybe even have it so you can customize each room again you get so much materials after a while. If you don't like that you can make it harder to get the love orbs which makes the game go by so much easier. I had everything and didn't even find the last material because i felt it wasn't needed.
You could also make it so that when one of the girls naturally goes in 100% heat they will appear in your room until you have sex with them or a few days go by. As it should be that it goes away or they will just masturbate until they lose their heat.