Default apps will be interesting, but the system has been so locked down to third parties for so long, I can't think of many.
Keyboard, for example. Ever since third party keyboards have been available, only a few developers have even seriously tried. There's Gboard (abandoned, of course. It's a Google product.) There's Swiftkey which Microsoft threatened to abandon and then immediately went back on but still failed to substantially improve.
Third party keyboards are awful on iOS and nobody seems to care to change that. Probably the only reason Google and Microsoft did it was for user data to mine.
So, can anyone name an actual third party keyboard that they use, let alone recommend?