Well, the answer to that is oh so obvious. NOT! First of all, you should consider what your pictures will be used for, and by whom.
The most important factors in a camera for personal use only are, of course, your budget and your opinion. Yup, if it is for personal use only you can forget what others think and base your decision on those factors alone. Generally, a quality point and shoot will be fine. As far as brand, there are a number of high quality companies that make really good point and shoot cameras. One factor to consider when buying a camera is that online reviews are worth about as much as the paper they are printed on. Wait, they aren’t printed on paper. Exactly!!! So when buying a new camera, buy it from someone you know will stand behind their product. That is one aspect you can evaluate online. If someone sells you a bad product, well quality control can’t catch them all. But it is what the company does about it when that happens that distinguishes the good from the bad choices. Read the bad reviews and see if the review was based on quality issues or customer service. Every company will sell a lemon on occasion, not every company will convert it into lemonade for you when that happens!
Now, on to the higher quality cameras used for stock photos, websites, enlarged printing, etc. My opinion is that when looking to buy a good camera you should not buy based on camera features. Really? Why is that? Well, first of all there are way too many companies that list way too many features to compare and make a reasonable decision based on features alone. Oh, but don’t buy a camera with a single attached lens. That is one feature that does matter. The technology for digital zoom is not that great, at least at this point. So you want a camera with interchangeable lenses. Which brings me to the real deciding factor, in my opinion. Buy a camera from a company that sells a broad line of quality lenses, and started doing so before last week when they introduced their new product line. Why do I say that? Because if you keep taking pictures for very long, you will end up buying a lot of different lenses, for different purposes. And you will end up wanting to get a newer, better camera sooner or later. But your lenses will be something you will keep and use for a long time. You will likely end up investing much more money in lenses than a camera. I am not a brand snob, who argues that my brand is better than yours based on trivial statistics and graphs. The truth is that most major brands of cameras are essentially equal in quality, with each holding a slight edge in one area of consideration or another. I use Canon, but that is more a matter of history and what I chose oh so many years ago. Nikon is also extremely good. And there are a few lesser known brands that are really good.
Ok, but what about the question of mirrorless vs. DSLR? Well, my answer there is that it depends on your budget. If you can afford a quality mirrorless camera and lenses at this time, they look to be the thing of the future. But in fact at this time DSLR prices, especially for used equipment of professional quality, are looking very cheap compared to historical trends. But if you can afford it the size and portability of mirrorless, along with the high quality images, make them a sure choice. Again, look for quality lenses. And if you want to stay with the same brand, look for the ability to use the lenses from your DSLR. I know Canon sells an adapter for about $100, and though I am not sure about Nikon and other brands I would be shocked if they don’t have the same.
So, what about sensor size? Is there really that much difference between full frame sensors and APS-C format sensors? Personally, I have one of each, and there is a small difference that becomes noticeable when I need to select a small crop area from images after the fact. But since I shoot a lot of nature scenes and try to do my cropping in-camera I mostly do small amounts of cropping on any individual picture. That is not always possible with wildlife, though, so sometimes I do regret not using my full sensor camera. But otherwise they both take great pictures.
How many megapixels should I look for? The answer again is obvious. Enough to do what you want to do with them. If you are looking to enlarge your images, all cameras of 16 Megapixels or more will probably be enough unless you want to enlarge to huge sizes.
Again, budget and desired use are the most important factors in buying a camera. Buy the camera that best fits your projected use and budget, and go take lots of great pictures!