The landing page should say: "We are building this product, but it's in the early stages. Leave us your email if you're interested to learn about its progress and availability before your competitors do. Early adopters will get discounts / free plans."
If this page gains any traction, subscriptions, interest expressed via the "Contact us" page, then the product is likely worth building past the feature list and a general idea of the architecture. You better have a prototype that will allow you to quickly demo some of your key promised features.