What about something like having fixed 64-bit instruction bundles, where the first 4 bits indicate if it contains 4x15-bit, 3x20-bit, or 2x30-bit instructions? This limits the combinatorial explosion in cases hardware needs to handle for variable-width decoding (effects don't cross 64-bit boundaries). One could reserve some of those 4-bit patterns for indicating instruction dependencies (a la EPIC/Itanium) and/or indicating extended functional units with completely different opcodes (similar to EPIC/Itanium opcodes having completely different meaning if marked as FPU instructions).
Presumably some of the 4-bit prefix patterns would indicate that the final instruction in the bundle was an immediate value instead of an instruction.
Alternatively, one could take a page form Sun's MAJC playbook and shorten the opcode of the first instruction in the 64-bit bundle by 2 bits to indicate 4x16-bit, 2x32-bit, 16+16+32-bit or 32+16+16-bit instructions in the bundle.
Presumably some of the 4-bit prefix patterns would indicate that the final instruction in the bundle was an immediate value instead of an instruction.
Alternatively, one could take a page form Sun's MAJC playbook and shorten the opcode of the first instruction in the 64-bit bundle by 2 bits to indicate 4x16-bit, 2x32-bit, 16+16+32-bit or 32+16+16-bit instructions in the bundle.