Unless the idea is very good and unique in some way, I don’t see much benefit in a desktop.
An AIO maybe… but the profit margins I would expect to be quite low, the device itself expensive and the sales volumes not as high (the desktop market has been stagnating for a long time).
A “normal” desktop computer definitely a bad idea: plenty of offer on the market and people competent enough to find out good offers are also likely competent enough to assemble or modify one on their own.
It could make sense something not targeted at the consumer market but, rather, at professionals and businesses. To companies, an AIO makes more sense and the right combination of components can make all the difference in terms of serviceability and lifespan.
To this regard, should Eve desire to pursue this path, I would actually suggest to start asking directly to professionals and influencers. For example I would expect LinusTechTips to be able to provide a very good “wishlist” for such product, considering their needs. If questions cannnot be asked directly, looking at some of their reviews of similar products and taking note of the good/bad points of the various products could be a very good starting point.