Spellskite seems good to protect Arcum, and can be sacrificed for a combo line if you have counters in hand
Seargent Tibbs, Curageous Cat and Duke of Weselton, Opportunist Official are one-cost cards that can help you establish yourself on the board right away, while Aladdin, Street Rat is a two-cost card that can Shift into Aladdin, Heroic Outlaw later on
Rikku, Resourceful Guardian allows you to force a creature through your opponents' blockers, as well as steal counters, and Lulu, Stern Guardian protects you by putting stun counters on your opponents creatures, as well as proliferating to keep them around
Clarabelle's five strength, six willpower, and two lore all get the job done, but the Keep in Step ability is the main draw
Marvels Spider-Man has additionally been confirmed to kick off a multi-year collaboration between the WotC and the juggernaut comic book brand.
I gathered all available Magic Online decklists from scheduled Modern events held between August 8 and August 14