Running on 16-bit systems like 286/386/486 in a
"ReactOS Essentials"
 (equivalent to a stripped-down XP) mode
 I am exprment with a dymatic translator betwin diffent CPU if it goes in
 the way as I like, it will posible runing any
 CPU in reactos so long the apps is writen for windows. 
386 and 486 are already 32 bit cpus.
I don't think porting reactos to 16 bit would be useful, because many
structures are for 32 bit numbers. Besides of this, most 8086/286 does
only have 1 MB Ram, a few 4 MB (the theoretical maximum is 16 MB for 286)
Porting it to PowerPC or Arm would be much more usefull ;)