Whatever you do, how many other characters you may add, Just please don't let away your focus from Grandma, coz she is the main Love Interest of this game. Please don't make this game as a Harem.
Or even if u plan Harem, by the game's ending, just make sure Grandma gets the "most number of lewd scenes" & also consider giving as a "Grandma Only Ending" by player's choices.
Sorry for talking and asking too much. It's soo rare and almost impossible to see a game with Grandma as the main.. So, got excited for the "stuff"..