LittleMsSam has a mod that I LOVE called "Next Door" right now it only has two next door types: Park next door and Cafe next door. Basically, if you add this trait and activate it, a fully functioning park or cafe will exist on your lot, so long as the requirements are met. I prefer this to mods (including hers) that just allow you to fill any vendor or NPC slots, because they don't attract the townies and townie behavior in the same way the "Next Door" mod does.
This is just a long winded way of saying the coding is definitely possible and a modder has already done it, and I would be very happy with similar functionality from the game. Even if only one additional "next door" type was allowed it would open things up dramatically.