In the Mahaparinibbana Sutta:
Is the metaphor.."No such direct personal knowledge, indeed, is mine, Lord, of the Arahants, the Fully Enlightened Ones of the past, the future, and the present; and yet I have come to know the lawfulness of the Dhamma. Suppose, Lord, a king's frontier fortress was strongly fortified, with strong ramparts and turrets, and it had a single gate, and there was a gatekeeper, intelligent, experienced, and prudent, who would keep out the stranger but allow the friend to enter. As he patrols the path that leads all around the fortress, he does not perceive a hole or fissure in the ramparts even big enough to allow a cat to slip through. So he comes to the conclusion: 'Whatever grosser living things are to enter or leave this city, they will all have to do so just by this gate.' In the same way, Lord, I have come to know the lawfulness of the Dhamma.
"For, Lord, all the Blessed Ones, Arahants, Fully Enlightened Ones of the past had abandoned the five hindrances, 14 the mental defilements that weaken wisdom; had well established their minds in the four foundations of mindfulness; 15 had duly cultivated the seven factors of enlightenment, and were fully enlightened in unsurpassed, supreme Enlightenment.
"And, Lord, all the Blessed Ones, Arahants, Fully Enlightened Ones of the future will abandon the five hindrances, the mental defilements that weaken wisdom; will well establish their minds in the four foundations of mindfulness; will duly cultivate the seven factors of enlightenment, and will be fully enlightened in unsurpassed, supreme Enlightenment.
"And the Blessed One too, Lord, being at present the Arahant, the Fully Enlightened One, has abandoned the five hindrances, the mental defilements that weaken wisdom; has well established his mind in the four foundations of mindfulness; has duly cultivated the seven factors of enlightenment, and is fully enlightened in unsurpassed, supreme Enlightenment."
castle-->Samsara
gatekeeper-->Buddha
or is it...
castle-->Sariputta's mind (or anyone's mind)
gatekeeper-->Sariputta (or anyone who chooses to guard the mind)
With either interpretation, I still don't understand the relevance of the metaphor to Sariputta's lofty claim -- that there will never be any Buddhas in the future with higher enlightenment. He just doesn't know that. He doesn't know it, but he speculates, the Buddha questions him, and he defends himself with a lofty metaphor that's great but doesn't seem to establish the point.
That is, a man who guards one particular castle, who has observed one particular gatekeeper -- this doesn't mean the castle is eternal or can't be improved, that the gatekeeper is perfect, he doesn't know all the other castles and all the other gatekeepers out there. In the absence of "Direct personal knowledge" (which Wisdom Publications' translation merely translates as "knowledge"), Sariputta's lion's roar really is just lofty... He comes to know the lawfulness of the Dhamma, but what does this have to do with the knowledge of future Buddhas?