I think a designer role would be fun, and it would help distinguish this game from the others. Being THE mayor never appealed to me all that much, but it would be nice to have changes on the town.
I'd choose mayor! I can't really imagine purposefully choosing to be just a villager, at least for my main file. And I.... am not the best at designing houses lmao. Though that being said, I think I'd prefer to have the mayor and designer roles mixed. Why lock half of the features off from yourself?
i'd love to be both the mayor and a designer! i really love designing houses in HHD, and also taking on mayorly duties in my town. maybe the villagers could post requests for their home to be professionally redesigned and they can provide you a budget, and you can work with that amount of money, plus any extra expenses you'd be willing to provide yourself. you wouldn't be able to pick from a full catalog like in HHD, you'd need to have had the items before you can purchase them again for use in a villager's house. it would take probably a day or two to complete the request since you'd need to order the furniture.
you can even expand their house size maybe! and as the mayor, maybe you can designate plots of land that villagers are allowed to move on to. you can make a little cul-de-sac that way!