There's a few possibilities I can think of:
- The doubled B and E you suggested
- Octaves on the G and D
- Modified Van Eps tuning: seven-string/low A (or B), octave on the G (if you like soloing further up the neck this'll give you a five-octave range)
- Taylor baritone: B-B or A-A, octaves on the third and fourth strings
- or you could keep it as a 2x4 8-string and try one of these:
- Long-scale octave mando (I'd try this first - could be a real cannon)
- Modified Irish bouzouki: GDAE with octaves on the lowest three strings (use a .007 for the high A)
- Modified Greek bouzouki: DGBE with octaves on the D and G)
Could be interesting...
