Hatsune Miku: Project DIVA Mega Mix just recently released on Switch, and SEGA is already offering a new round of free content. Two different t-shirt packs are up for grabs, which you can read up on below.
T-shirt Design Pack 1: includes the winning entries of the t-shirt design contest that was held in Europe and North America
T-shirt Design Pack 2: includes the t-shirt designs that were offered as purchase bonus at various retailers across Japan
If you want to check out more pics of the various t-shirts in these free packs, you can visit this link.