The A350-900 and A350-1000 are natural replacements for Iberia's A340-300s and A340-600s. The A350-900 would also complement the 787-9 for British Airways' 777-200ER replacements, though the 787-10 would offer more capacity for TATL missions.
As a 747-400 replacement, the 777-9 has the advantage of being able to more closely match the capacity of both the low-J and high-J models than the A350-1000 can. The 777-9 can use the same Economy Class hard product as the 747-400 in the same 10-abreast configuration and the additional 3 meters of cabin length would allow BA to roughly fit 16 additional Club World seats on the main deck, which would mostly make up the loss of the 20 seats on the 747-400's upper deck.
So I could see IAG operating all models of the 787 family (-8 | -9 | -10) along with the A350-900, A350-1000, 777-9 and A380-800.