Tetris 99 has a bunch of things you can unlock, if you play enough to collect "tickets". Certain actions, such as a number of KOs or a number of lines cleared, can win you tickets. You can redeem tickets for small upgrades.
A notable one I found recently is the Super Mario Bros theme, complete with overworld background and music.
Sadly, the middle part of the theme isn't present, so the music loops quite quickly and becomes a bit repetitive.
Still, it's a nice change from the standard look and feel of Tetris 99.