I am saddened to say, but everything that made the Amiga great when it was launched seems meh by today's standards.
Amigas had preemptive multitasking before any other home computer. Today it's the norm. Amigas could switch video modes every scanline. Today, even the concept of a scanline takes a while to explain to someone born in the 90's. The same happens with the careful NTSC timing ("what's NTSC, grandpa?") that, ultimately, doomed the machine to low-resolution graphics until it was too late. Hardware accelerated graphics (their blitter was cool) is also the norm (and many orders of magnitude faster now). We don't even talk about audio quality anymore.
Everything that defined an Amiga is commonplace. To experience a UI that looks like a cross between Gnome 2 and BeOS running on a processor that once replaced 68Ks is not what would define an Amiga today.
So, what I would expect an Amiga for the 2020's to be?
I'd imagine, for starters, a uniform CPU/GPU architecture. Lots of cores accessing a single memory pool (let them have some scratch memory for themselves to save bandwidth) running the same software doing the job of both CPU and GPU. Make this CPU/GPU hybrid monster capable of combining multiple 4K inputs piped into textures and composite them in real time on any display surface available. A radical idea should set the tone.
Maybe, for the GUI, explore the idea of depth to convey information about windows. A simple sensor could locate your head and adjust the screen perspective accordingly. To the programmer, the GUI should be simple. Reminisce about Plan 9 a bit. If the head detection thing is capable of dealing with hand gestures, have a couple gestures to zoom in/out, switch task/workspace and so on. Make the visual language elegant for the 2020's. Let go of the 80's.
Embrace the idea of "the network is the computer". Again, think Plan 9.
Obviously, make the OS developer friendly, not only to develop for it, but for other platforms as well (whoever uses this computer needs to be able to work). Don't bother with the Amiga CLI - we have better stuff now. Ports of Clang/LLVM, OpenJDK, the usual browsers and, of course, Emacs, are the bare essentials. Make it POSIX so it's easy, but favor languages that express concurrency and multiprocessing easily because you'll have a whole lot of cores.
Obviously it should be able to read most common filesystems, even if its native filesystem is something alien. Snapshots, implicit RAID, part of the computer address space... Something new, please. Maybe don't have a disk and treat RAM as cache for a persistent store. It'd need to be radical or it'd be just another cute computer.
Ditch the mouse. Use a trackpad. Just make it right (as good as Apple's).
The system enclosure can't look like a PC. If at all possible, build it with no cables - edge connectors to drives, power, external ports. If it's pretty enough, you get the chance of using transparency to show it off. Again, 2020's, not 80's.
Use the ball as soon as possible at boot, at the highest resolution the display allows. Use the bounce sound to signal successful POST. 2020's, but still Amiga.
Finally, the keyboard. Whatever you do, don't make it a generic (or worse, cheap) PC keyboard. An Amiga should feel different (even if the original was so-so). From layout to switches to keycaps, it'll have to convey uniqueness.
That ended up longer than expected...