I think I'd find the idea of using the airport to get to a shopping centre really exciting; the island is great but can be quite claustrophobic at times. You could possibly find other villagers in the shopping centre, or just see more animals to talk to but not invite back. Here would be my ideas, just off the top of my head:
-You wouldn't be able to bring things like sharp tools or bugs or fish into a shopping centre! You have to leave them at home.
-Flick and CJ would have their own pet shop! Because of the reason above, you can't sell them anything, to encourage you to wait until they visit your island; however, you CAN buy lots of stuff from them! They would sell things like objects such as big tanks for your fish, a MASSIVE tank if you want to put all your pet fish together (ditto for the bugs), little ornaments to put in the tanks/cages. They also sell bait that you can buy (you can't buy bait from Timmy and Tommy, you HAVE to buy it from CJ) or maybe an object or bait that would encourage rare bugs out of the earth (again, you have to buy this from Flick). You can also buy better fishing rods and nets, as well as look at calenders to see when the next Fishing/Bug Tourney is. You can even tell Flick and CJ what day you would like them to visit your island and they'll check it in their calender!
-Kicks would have his own shop, as well as Shampoodle. Not much to say about that, except that Kicks now has the space to offer a far wider range of shoes and bags, as well as maybe other accessories. Maybe he would also offer you the chance to make your own bag!
-A cinema! Maybe this is wishful thinking but could it have been foreshadowed in one of the very early New Horizons trailers? If anyone ever played Club Penguin, I'm thinking of something like The Night of The Living Sled, a selection of tiny films the developers have made beforehand; it was a big thing in Club Pengiin to go and watch it with your friends and pretend to buy popcorn and drinks, even though it was only like 30 seconds long. OR MAYBE it could even showcase the films you've made on Harv's Island, and if you have Nintendo Switch Online, you can see films other players have made if they make it public!
-A supermarket, where you can buy ingredients for cooking! I'm not talking about getting rid of the farming aspect altogether, you need to grow and forage your own *food*, but the supermarket lets you buy harder to find naturally things like sugar, salt, etc. Maybe even cooking supplies are sold there for quite high prices, like pots and pans, and certain recipes are unlocked once you have bought all the cooking supplies needed to make them!
-An art studio, where you can more easily look for *paintings and canvases* specifically that other players have created and made available for public use
-Digby and Lottie now work together at the Happy Home Academy! They can offer more extensive home options, like more fencing, and more outdoor customisations. They can also offer exterior furniture that hasn't been seen before.
That's it off the top of my head....I've made myself far too excited now.