A 200-400 5.6 won't cut it.
Want a 200-500 5.6 for some 700mm action with a 1.4 TC that will look better than the Tamron 150-600's 600mm @ the same f/8.
The problem is, a 500/5.6 has an aperture of 89.3mm - much larger than the 71.4mm of the 100-400L. Canon would charge a lot for that aperture - probably well over $4,000. Since the 100-400L currently sells for $1,500, a $4,000 lens wouldn't exactly be a replacement. When the 70-200/2.8L IS I was selling for $1,700, the replacement went for $2,500. I expect a similar jump in price (~50%), not a much larger ~150%-200% jump. Therefore, a 200-500/5.6 or thereabouts would not be a replacement for a 100-400L in the lineup, but rather a new option.