Rachel Rofe has a proven system for coming up with product designs that will sell. An Idea File is always handy for Rachel Rofe. It could be a notebook and pen that you keep around, or a TextEdit document you have minimized on the Mac. This will help you capture inspiration whenever it strikes. Her first instinct is to look for word-based designs. These are quicker, easier, cheaper, or even free to create. First, go to Google Images. You can search for the niche you are interested in.
Let's say your dropship on-demand product is a coffee cup. Search "funny coffee cups for dad" to see what results you get. You can add anything to your Idea File. You can do more searches. Keep jotting down more ideas. You can do this as many times as you like. It takes only a few seconds, and it doesn't cost anything. You can also use a few Pinterest-based tools if you feel you have exhausted Google Images. Pinsearcher or PinterestSort.
Both are Google Chrome extensions. You can choose the one you like most, then go to Pinterest and search for keywords in your niche. Then, consult the plugin. It will show you which images have received the most pins. It's likely that those products are popular because many moms will pin things they want to buy later. It might be worth modeling these designs.
You can also search within Pinterest, and then use the built-in feature to sort by how many repins. Rachel searched for "funny knitting shirt" and found a tank that said "Knitting takes yarn balls". You could slap that on a mug and probably do pretty well. It is easy to come up winning concepts.
You can also do some manual research on Amazon, Etsy and Walmart to find top sellers in your market. You should look for products that are word-based and have hundreds, if not thousands of reviews. The data will tell you what people like. You can use paid tools to aggregate this information, speed up the process and make it more beautiful.
No matter what your preferences, you should be capable of creating a large number of viable ideas within a short time. It is ethical to invent your own variations. Do not just copy others' words. It's actually not difficult. Do not overthink it. You can just look up synonyms and change one word at a time. No need to second-guess.
You might find other niche-appropriate sayings. Rachel was looking for clever lines for teachers when she came across a Tshirt that said: "I do this because the money is good." That's pretty clever, right? She applied it to firefighters. "I do this for money," she said to them. Talk about an easy way of scaling up your print-on-demand business.