Eighteen megapickles is really a great size. It gets you 13" x 19" prints at (almost) 300 ppi, which is as big as almost anybody who uses APS-C camera ever prints and is more than enough image resolution for any application you care to name. It also gets you 24" x 36" prints at (almost) 150 ppi, which is bigger than 99 44/10% people with an APS-C camera ever think to print, and that's a higher resolution than your computer monitor. Or, it leaves you plenty of room to crop.
If you think back to the film days, almost nobody would think of printing even 135-format images at those sizes, and yet you can do it with ease with at 18 megapickle APS-C camera -- quite remarkable, really.
So, more would be better right?
Well, not really. The standard 18 megapickle cameras are already a bit of overkill, though by an amount comfortable enough for both room for error and for those who push the medium beyond its intended design limits. You're not going to be able to do all that much more with more megapickles at your disposal, except maybe crop a slightly bit more aggressively when you're shooting at base ISO, or maybe make 17" x 24" prints instead of 13" x 19" prints, maybe. But the tradeoff is larger file sizes, which means fewer shots on the card, and either a slower framerate and smaller buffer or more expensive onboard processing -- as well as slower processing in Lightroom / Photoshop / whatever...plus bigger hard drives, and all the rest.
All that overhead just so that a minuscule fraction of a percent of the userbase can push the camera even farther beyond the limits of sensibility?
What on Earth for?
No, if you're not getting the image quality out of APS-C that you want / need, do what photographers have done since the beginning: move up to a larger format size. And you can do that once and still use (most of) your lenses. And if the twenty-something megapickle full-frame cameras still aren't good enough, there's medium format up to a hundred or so megapickles, and then large format beyond that if you need even more.
But, realistically, damned few people even push the image quality limits of APS-C, only a small percentage push 135, and you probably don't have to take off your shoes to count those who push medium format's limits. Yes, there are absolutely advantages to the larger formats, and, no, you're almost certainly not one of the people who could put those advantages to good use.
Indeed, unless your printer takes ink by the gallon, 18 megapickle APS-C is perfect for you, with all the image quality you'll ever want or need without any of the headaches that come from bigger systems.