Acting for Two: In Mission 12, Yoko discovers she has a Doppelgänger, a Chinese actress named Angie Sue. They look exactly alike except for their hairstyles. Naturally, they're played by the same actress.
In Go-Busters Returns, the voice actors for the Buddyroids all get to have on-camera roles (while still playing the Buddyroids as well). Cheeda Nick's actor plays a man who gets lost, Gorisaki Banana's actor plays a school teacher, and Usada Lettuce's actor plays the alternate team's Sixth Ranger.
And of course, there's the fact that the vast majority of the cast does this in Returns, playing both their normal characters, and their alternate-universe counterparts.
In Mission 17, we are treated to Beet Buster (whose actor previously did Magi Yellow) saying "maji de ikuze"note meaning "Time to go seriously" while performing the Magiranger hand gesture. The Maginote shorthand for magic/majinote serious pun was often used in Magiranger.
#30 has Jin running over Messiah with BC-04, bringing to mind how another golden mecha back in Magiranger (MagiShine's Travelion) ran over one of the villains. It was The Dragon that time; here it's the Big Bad!
This allusion is taken to its logical conclusion in Go-Busters vs. Gokaiger, when thanks to the Go-Busters' ultimate power, Jin (and J) get to pilot MagiKing. Jin even points out how it "feels familiar".
Filmroid (the Metaroid for Mission 20) is voiced by Kenichi Suzumura, whose mannerisms should remind you of a certain Imagin.
The DS game is released only in Japan and in South Korea.
Prop Recycling: Other than the fact that Megazords use parts of the Metaroids' costume, any Metaroid with 2 at the end reuse previous costumes, especially if they are repaints. Mission 45 opts to hang a lampshade on it by reusing the Eraserroid prop as Omochiroid, using the same plot point of it piggybacking on Hiromu's back to go somewhere, and then having J and Hiromu point out that this whole plot seems familiar.
Reality Subtext: The whole premise of Enetron itself, thanks to the protests against reliance on nuclear energy, after the Fukushima nuclear plant failures in the wake of the 2011 earthquake.
Similarly, the finality of everyone from the energy center sent into hyperspace being Killed Off for Real could be seen as a coping mechanism for all the lives lost in that earthquake. Compare Mahou Sentai Magiranger, which starts with a Disappeared Dad, the remaining parent becoming a Missing Mom by the first episode, and yet both turn up alive and well by the end.