A few ideas for possible future additions:
rival mode - rival npc and player fight rather than coop like in the third episode
breed off- extended vs mode with scoring based of impregnation count.
gallery to replay unlocked animations
toggleable cutaway or side screen that plays animation instead of between missions
more mission types: example; fuck in (room type) for (period of time) hybrid of the current hide and room objectives.
endless mode (art and animation heavy) creates endless stages you must complete to see how far you can go.
Extended mode: more objectives in the base missions, (add a toggle for sloppy seconds for missions like episode three)
basically you have a good game so far with potential for future development if continued.
edit: few more ideas not sure how many are possible but I thought I'd add them to the list
versus mode: vs against the husband instead of a rival)
orgy episode (2-3 ally npcs, 2 possible fuckable npcs, objectives only no rival npcs.)
shadow breeder challenge mode: impregnate fuckable npc before husband npc.
fog of war (hard mode) vision aimed toward last player movement (don't see any npcs in fog at all.)
rival+ or rivals (rival mode but multiple enemies.
fuck fest - free mode (more or less interactive gallery use only unlocked animations from main story)