I think in a lot of aspects it will be a sequel. The best way to keep Stuart as the main character is simple you very rarely have all of the old cast there all at once. Like maybe have Penny come in without Leonard to pick up a comic he ordered she only say 4-5 quick lines then she leaves. Maybe have raj come in and browse and way in on whatever Stuart and his GF are talking about. Howard sneaking into the store for some time away from the house. With all the characters dropping lines the tell the audience how there lives have been like how Penny has adjusted to being a mom after not wanting kids. How Howard been working on or got his doctorate. But never allow all the characters to take over always have what they have to say about there lives fit in with whatever the conversation is.