This rumor has now been confirmed, you can find more details here.
We’re going to mark this down as a rumor for now, but work is definitely underway to expand Minion Park at Universal Studios Japan. The expansion has yet to be announced, but walls have been up for a month or two now in significantly large and unused part of the themed land. What could they be building? Possibly a quick service dining location, or maybe even the Silly Swirly spinner attraction like the one at Universal Studios Hollywood. Based off of Google Earth measurements, the land area in this expansion going on at Minion Park is large enough to fit the Silly Swirly ride. Until this expansion is announced, or officially opens, we are going to leave it marked as a rumor.