It would be an interesting lens option. Hopefully sharper than the EF 24-105/4 (and my experience with the Sigma GV lenses says that after calibration it probably would be). But at the same time bigger filters and increased weight (and possibly higher cost) would be negatives.
We've been seeing bigger front element sizes on lenses everywhere and I take this to mean that lens manufacturers recognise that with the advent of digital photography people that own *SLRs have become much more interested in IQ.
So if you want smaller lenses with the accompanying smaller filters but you don't want to compromise IQ then you need to move to m4/3.
Or in other words, it isn't possible to deliver a 24-105/f4 for 35mm cameras that is lighter, smaller and has better IQ than the existing lens.