To the best of my knowledge, the promise that Ephraim and Menashe would become great is the following verse (Bereishis 48:19 from Chabad.org):
יט. וַיְמָאֵן אָבִיו וַיֹּאמֶר יָדַעְתִּי בְנִי יָדַעְתִּי גַּם הוּא יִהְיֶה לְעָם וְגַם הוּא יִגְדָּל וְאוּלָם אָחִיו הַקָּטֹן יִגְדַּל מִמֶּנּוּ וְזַרְעוֹ יִהְיֶה מְלֹא הַגּוֹיִם׃
19. But his father refused, and he said, "I know, my son, I know; he too will become a people, and he too will be great. But his younger brother will be greater than he, and his children['s fame] will fill the nations."
Rashi there explains:
he too will become a people, etc.: for Gideon is destined to be descended from him. [Gideon] through whom the Holy One, blessed be He, will perform a miracle. — [from Midrash Tanchuma Vayechi ]
But his younger brother will be greater than he: for Joshua is destined to be descended from him, [and Joshua is] the one who will distribute the inheritances of the land and teach Torah to Israel. — [from Midrash Tanchuma Vayechi ]
The end of that Midrash Tanchuma says:
...ואימתי נתקיימה ברכתו בחנוכת הנשיאים שהקריב אפרים ביום ז' ומנשה ביום השמיני
...And when was this blessing fulfilled? When the Princes inaugurated [the Mishkan]. Because [the prince of] Ephraim was on the 7th day, and [the prince of] Menashe was on the 8th day.
As such, it seems to me that according to Midrash Tanchuma, that particular blessing has already been fulfilled.