I think most potential customers won’t mind if it is similar in appearance to other lights, especially popular styles like the ubiquitous C8 style or the Eagle Eye X6. But, you can take a style like that and make some small change, (hopefully improvement) and make it your own.
Maybe take a classic that hasn’t yet been updated to a 21700 size, and do that, and maybe add one or two very small aesthetic changes, and you will have a totally new light that’s never been done before!
Another possibility is take two different designs that have had some amount of popularity, and merge them somehow, taking some design elements from each of the ‘parents’ to make a nice looking new light.
Here’s another that will be a sure winner: Look at lights that have been popular over the years, but for some reason were discontinued while still fairly desirable. I’m not talking about lights that buyers got tired of, and caused sales to wane. I mean lights that people still liked, but the manufacturers decided to move on and drop them. Resurrect an old design, update it with new internals, and maybe make very small changes to the outside to make it your own design.
One thing a lot of people might never think about: look at lights that are ‘too cheap’ but are otherwise a decent design. There are lots of lights that never become truly popular only because of how badly they are made. I’ve seen reviews here on BLF of lights that looked good, but were found to lack inner quality. Those lights most people wouldn’t know about, because nobody wants to buy them. But, if they were made well, the design might be irresistible! You can take the design, improve the quality, and release a unique product that people will love. If you just look through the massive collections on sites like Banggood, Gearbest, AliExpress, and Amazon, you can find plenty of these.
Last idea from me: Read reviews - lots of them! Find out what people like and what they don’t like about the lights we already have available to us. Take those ideas and make a new light that improves upon those existing designs based on that ‘feedback’ in the reviews. Read through the comments below the reviews too. Even the audience has some insightful things to say at times.