I think I remember them saying that there'll be something like Eureka coming during the 5.x patch cycle, but it may not be anything to do with the relic.
We also have Ishgard restoration in 5.1 which I'm actually pretty excited for honestly, and will probably be a decent way to level up gatherers and crafters. Though this honestly looks like it'll likely replace custom deliveries (or be the 70-80 custom delivery content).
There's also the island thing that Yoshi-p was talking about where we can raise monsters, but I've only heard him mention it once so who knows.
So a rough patch schedule according to past releases (112 days between) will be:
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5.1 [Oct 22nd / 2019] - MSQ, 24man, Hildibrand, Beast Tribe 1, Ishgard Restoration, 1-2 Dungeons, Ex Primal, 0-1 extreme content
5.2 [Feb 11th / 2020] - MSQ, Eden, Hildibrand, Beast Tribe 2, New Primal+ex, Relic quest, 2 Dungeons, New Tomestone
5.3 [Jun 2nd / 2020] - MSQ, 24man, Beast Tribe 3 (DOH/L), New Primal+ex, 1-2 Dungeons, 0-1 Extreme content, Relic stuff
5.4 [Sep 22nd / 2020] - MSQ, Eden, New Primal+ex, 2 Dungeons, Relic stuff, new Tomestone
5.5 [Jan 12th / 2021] - MSQ, 24man, Hildibrand, Alliance Beast Tribe, New Primal+ex, 1-2 Dungeons, 0-1 extreme content, Relic finale
6.0 [May 4th / 2021 (This will likely be put back to June or even July).]
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The content will also be peppered in with random shit, like new stuff they're developing currently. They constantly pepper in updates to the gold saucer and random QoL stuff as well as random things like deep dungeon and treasure map content. That's just a brief overview of the major stuff likely added in those patches.
These could be off by the factor of a month, but they generally keep to a 112 days (16 weeks) patch cycle. There's a solid chance the September date could be put back an extra week, and the 6.0 release will very likely be delayed because it's a new expansion and it falls really close to Golden Week in Japan.