There are different ways one could approach this question. It seems like others are doing a great job giving you interesting synonyms for first - any of which could carry your intended meaning.
So, instead of repeating what others are saying I am going to take a more business-focused approach to answering this questions. To begin with, most experts advise businesses to focus on what the market needs not on what product they have, for the very simple reason that what is going to sell is what people want. Sometimes a startup might begin with a product that someone developed for his own use, but then realized that others might want to use that product, too. But even then, the focus quickly turns to who wants this product and how to enhance this product to give people exactly what they want, or at least the best cost-benefit ratio to the seller of what people will be willing to pay for? The business through its products is really meeting the needs of a certain market niche, and the ability of a business to meet the needs of a certain market niche can be described as that business's core competencies.
core competency: a defining capability or advantage that distinguishes an enterprise from its competitors. - Google
The corresponding word for core competencies when referring to products is core products:
Core products are a company's products which are manufactured from the
company’s core competencies. These core products are then integrated
into other products by the same company or a company which produces
the main end product from this core product and the end products are
marketed to the end consumers.
Generally core product gives the main benefit of the product. But many
more things are added to it to add value to the product. This leads to
the conversion of core product to actual product and then augmented
product. So, augmented product gives final complete product to the
customer.
So, from the link above, one can see examples like Microsoft - core product being the Windows OS, but an actual product might be Windows 2015, and then there are all the augmented features which Microsoft also includes which ends up being part of the final product that Microsoft ultimately sells to the customer. Augmented features can include many upsell items like extended warranty, extended or premium customer support, etc.
So already you are probably understanding that one cannot simply say other products. Are you talking about the same core product but different versions of the actual product like how Windows 2014 might differ from Windows 2015? Are you talking about different augmented features that will ultimately be a part of your complete product offering? As you can see, in many cases new products are really a part of expanding the product offering around the existing fundamental, core product.
Or, are you talking about other products that are outside the existing core competencies which the business will have corresponding to its first core product? This is generally not advisable as successful businesses tend to cluster products around the same set of core competencies and businesses tend to struggle when they try to offer products outside their core competencies.
New Product Development (NPD) will take in to account the consumer’s
preference for benefits over features by considering research into
their needs. NPD aims to satisfy and anticipate needs. NPD delivers
products which offer benefits at the core, actual and augmented
levels.
So, my answer is that a business's initial product is also defining its core product in that the business builds its initial core competencies around this initial product. It is important for businesses to consider whether additional, future products are complementing these core product competencies or whether these future products are outside of existing core competencies, which a business will generally want to avoid.
