Just going off of Nordhaven itself, I am thinking of adding a park for that lot right next to the waterfall and restoring the fisher's market with a Marketplace. (Thus finally adding all three of the Eco Lifestyle lots to each world!)
But as for business ideas, I do literally have a family-interest-in-venues document already set up for all of my households for the wealthy ones. So perhaps...
I can see Katrina Calinte making a move to own the lounge in Oasis Springs. It's her performing space after all.
A retail, not small business, for the Flecther family. As an electronics store. Since I want to sell more than just robotics crafts. Computers and such as well!
The Feng household in San Myshuno established a library over that lounge in San Myshuno... and now they will charge by the hour! How devious.
This wouldn't even count all of the lots across worlds in which I would at least place a whiteboard down to have lectures with, nevermind actually doing some classes. Foxberry's rec center needs to have a place to take robotics classes! (And I suppose the same for Research and Debate on the other side). Could also see the Rec Centers I have around the worlds being set up for lecture hall areas. And libraries as well!