I've posted on this before, but personally, I like the idea of coating weapons/arrows and dipping same, but would love to see some limitations, including some connection with classes.
Poison seems most appropriate for rogues, rangers, druids and perhaps certain casters with alchemical skills.
Preparing your weapon with poison prior to combat should be possible if the appropriate character has poison in their inventory - The poison should be active for whatever period is determined (2 rounds at the moment) but need to be reapplied once that period is up, once that combat set piece is up, or next rest - whichever applies. Normally I would apply the same rules to arrows, but as we don't have normal arrows in BG3, it should be applied to the bow, as if applied to an arrow.
Grease, Tar, alchemists fire or whatever you choose to call it should also be limited, perhaps to steel weapons, used by fighter/clerical classes and maybe allow rogues to be the only class that can use both poison and the fire causing element. Again it should be use limited as above (number of turns/end of combat set piece/next rest. Fire applications should require the additional action of 'dipping' to set the substance coating the weapon on fire.
Wooden weapons should not be flammable - excluding bows, where the action of coating the bow is actually coating the metal arrowhead - again only because we currently have arrowless bows. Dipping of previously prepared 'flame' weapons can continue as already in place - bonus action or whatever.
There is no justification for imbuing weapons with any other damage - frost, lightning, toilet water - other than through a spell.
Weapons magically enhanced with some form of damage should not be enhanced by chemical fire means - let the damn magical ability interfere with coating it.
Poisoning a magically enhanced weapon should be solely within the purview of a higher level rogue, and solely for personal use.
Amount of damage for poison/chemical fire enhancement should be less than than provided by similarly magically enhanced weapons - the classic flaming longsword for example.
