Takedowns on enemy wards cause friendly Zombie Wards to sprout from their corpses. In addition, damaging a champion after destroying a ward inflicts bonus magic damage.
Proccing Comet on Rumble with either [E] or [R] is likely to result in a hit thanks to the slow. If opting for Rylai's [Q] should become reliable as well. Thanks to Rumble's mana-less, try to abuse Comet by landing as many abilities as possible while it's on cooldown.
Nullifying Orb allows you to trade aggressively into AP matchups where you might otherwise risk dying.
Absolute Focus is a great rune on Rumble when playing aggressively and poking in lane with [E]. Teleports bottom will benefit from the extra AP when looking for kills.
For a more reliable option, take Transcendence for some free early CDR.
Scorch is rune that is heavily abusable in lane by Rumble with [Q]. The damage scales decently on him as well since he likes to stack Magic Penetration early.
If you think the game is going to get stalled out into 30-40mins+ take Gathering Storm instead.
Cheap Shot grants you a significant amoutn of increased damage thanks to your ability to repeatedly apply slows from [E] and [R].
Zombie Ward offers you a ton of extended lane safety so that you're able to shove and harass your lane opponent.