Essential

Serration
Split Chamber
Point Strike
Vital Sense

Hard to Replace

Heavy Caliber - 60%+ increase to overall damage on top of Serration and Artemis Bow does not suffer from the accuracy penalty
Vigilante Armaments - Increase average number of shots from 13.3 to 17.5 for 30%+ increase to overall damage and increased number of status procs

Elemental Mods

Only space left for two 60/60 mods remain. This actually ends up being the best use of 2 slots if maximum Corrosive procs are desired. Can be switched for any secondary elemental damage, although at time of writing corrosive is most useful for high level content.
Replacing one with [Primed Cryo Rounds] gives just under 50% increase to overall damage, but needs at least 2 more Forma and then will be stuck with an inflexible Vazarin polarity.

Extra Forma

1 Extra Madurai polarity (on Point Strike/Vital Sense) will allow one 60/60 mod to be replaced with a 90% elemental mod. This reduces the number of status procs slightly (less status chance x more % corrosive) but boosts overall damage slightly (~14%). Will also allow any primary elemental damage (e.g. Heat only) for greater flexibility. Generally a favourable tradeoff.