Degenerate versions of the deck pack all the red combos, like Kiki-Jiki, Mirror Breaker and the pile of cards that go ham with it, Flameshadow Conjuring plus Worldgorger Dragon, and a dozen or more others
At least with you, we know theyll look great
Harmonize Next up is a card that seems to have been chosen for flavor rather than function
It turning on Mox JasperMox Jasper early in a deck that would have multiple other reasons to play it is just gravy
When a new one is added, discard the old one and its effects
One of the biggest challenges of building a sideboard in Modern is that there are are something like 30 legitimately playable decks and at least another 30 that are close enough to being playable that you'll run into them on a regular basis, both on Magic Online and in the paper world