i could get behind a relic +3 weapon. and if done right, only one is needed. make it a d8 weapon that is usable by both bard and cleric. currently, long sword is barbarian, fighter, paladin, ranger, and bard; while heavy mace is barbarian, fighter, paladin, ranger, and cleric. the only real difference is flavor. so make something that can be used by all those classes and let any similar UR weapon trade in for it.
as for the missing classes:
monk; if they get a relic weapon, it should be unique to monks.
wizard & druid; if they get a relic, why would they want it to be a weapon? it should be something with special abilities.
rogue; would need something better than a simple +3 weapon. they have nightshades short sword, which almost has a +3 damage wheel, it is just missing one point of +to-hit, and giving up the bonus sneak attack damage probably isnt worth trading into a generic +3 weapon...so, again, needs a special item.
this does bring up an interesting question: will we see class restrictive relics and legendaries? i hope so, because i like the variety that class specific items bring.