I always thought the family of the local GP would be good. I've had this idea for years now: a middle-aged female doctor with her elderly mother, husband and two children, with two more off screen to introduce later. A sort of modern family where this couple don't actually share blood with all the children, they're products of previous relationships/marriages. Making the mum the doctor would immediately involve her in the community, and having the children as products of previous relationships means there's a lot of scope for stories. Plus, the elderly mother - I've always thought it'd be good to have a regular character with dementia (but I guess the show has that right now with Sylvie).
Another idea I had was a gay couple in their twenties who have been together since they were fourteen and have only ever been with each other. A really strong relationship, and a couple who are very secure and proud of their sexuality. I think sometimes in soaps they very much rely on the 'struggling to come out' story, which is absolutely fine because there are a lot of people out there struggling to come out, but sometimes I think wouldn't it be a better message to those people to see a secure and proud gay relationship on telly, showing there's absolutely nothing to be ashamed of? I just think that particular route hasn't been considered before in great detail. These two would arrive with a big story putting them right into the heart of the action, in a story that has nothing at all to do with their sexuality.