Relics should feel like a journey. They should be introduced early, ideally at the very start of an expansion, with regular updates to keep you going across an expansion. The weapons themselves should feel iconic to obtain, and should have a bit of unique backstory to each of them. The ARR relics are still unmatched in this regard, both in terms of their names (Excalibur, Ragnarok, Sasuke's Blades, etc.) as well as their appearances.

The actual nature of the grind itself doesn't matter very much. Many relic steps do boil down to collecting a set number of tomestones or running DF/Fates a set amount of times for light/memories/atma/crystals/clusters. You can get a bit creative with this, like the Zodiac book step, but then it gets a bit prescriptive. The idea of sequentially adding stats over time by completing books was an interesting concept, though. The Resistance Weapons approach was pretty good, giving you the option of working through the exploratory zone or to do the usual DF content. Both Eureka and Bozja are surprisingly active, even now.

One thing that would be nice would be adding some form of storage or display for accumulated relics, especially if you've managed to collect all the relics in a given set.

At some point, I'd like to see a relic weapon set/exploratory zone draw reference to the Sealed Weapons of Kuza, from FFV.