I can't really comment on the tradeoffs between specific ISAs since I've mainly worked on micro-arch research (which is ISA agnostic for most of the pipeline).
As for the questions on research into looking at decode complexity v instruction density tradeoff - I'm not aware of any recent work but you've got me excited to go dig up some papers now. I suspect any work done would be fairly old - back in the days when ISA research was active. Similar to compiler front-end work (think lex, yacc, grammar etc..) ISA research is not an active area currently. But maybe it's time to revisit it?
Also, I'm not sure if Huffman encoding is applicable to a fixed-size ISA. Wouldn't it be applicable only in a variable size ISA where you devote smaller size encoding to more frequent instructions?
As for the questions on research into looking at decode complexity v instruction density tradeoff - I'm not aware of any recent work but you've got me excited to go dig up some papers now. I suspect any work done would be fairly old - back in the days when ISA research was active. Similar to compiler front-end work (think lex, yacc, grammar etc..) ISA research is not an active area currently. But maybe it's time to revisit it?
Also, I'm not sure if Huffman encoding is applicable to a fixed-size ISA. Wouldn't it be applicable only in a variable size ISA where you devote smaller size encoding to more frequent instructions?