I’m not sure. I know they’ll be adding in older non-villagers with updates and events, though, which makes me think that any new characters would be introduced the same way.
If Nintendo wanted to go the DLC route, they could; I’m sure people would be willing to buy it. I’m really hoping that they won’t, though. No Animal Crossing titles have done it so far, so I’m holding out hope that New Horizons won’t be any different.