There are overhauls/mods already that take crafting to T6 (and higher).
Legendary items don't need a "Tier". They would be unique (or super-rare) items that are Legendary through their given properties/stats/built in mods.
Example: Legendary sniper rifle - "OSOK" - One Shot One Kill - Entity damage 300, effective range 200 (plus player buffs/perks). Built in 8x scope, extended magazine, suppressor, bipod.
Done. No crafting necessary.