If we are takling 'classic' Amiga then:
1) New replacement mobo's for current models - preferably including some modern ports (proper VGA & USB) & maybe somewhat improved basic capabilities.
2) New replacement plastics.
3) Then, if the above goes to plan complete new systems in whatever form seems best for 'today'.
4) It needs to be cool.
If we are talking 'new' Amiga then:
1) It needs to run on cheap available hardware - X86 PORT!!!
2) It needs to have at least the basic apps any 'modern' OS would boast which themselves support (near enough) modern standards.
3) It needs to be Amiga-like & make a fair fist of running 'classic' apps + games.
4) IT NEEDS TO BE SUPPORTED!!! Which for a 'minority platform' probably means free / open source...
...AROS in time..?
5) It needs to be cool.