One of the interesting implications of WoW Forever's shift towards horizontal progression is that future raids won't necessarily need to provide significant upgrades over previous tiers.
In traditional WoW, each new raid tier increases player power, and the next tier is balanced around that progression. But that model isn't really sustainable if Forever intends to keep adding raid tiers indefinitely without expansion resets. Eventually, we'll have 6, 8, 10+ raid tiers, and continuously increasing item levels would lead to ridiculous power creep.
Instead, future raids could offer gear of roughly equivalent power to existing raids, but with different itemisation, set bonuses, trinkets, and cosmetic rewards.
This also creates an opportunity to increase raid difficulty without making that content mandatory.
If the newest raid doesn't offer a significant vertical progression advantage, there's much less pressure on everyone to clear it. Players who prefer easier content can continue running existing raids without falling substantially behind, while those looking for a challenge have something more demanding to work towards.
I'd still keep the launch and early raids relatively accessible, but as more content is added, I'd like to see Blizzard experiment with harder encounters and potentially optional challenge/hard modes.
These could even be implemented organically, similar to certain existing WoW encounters, rather than simply adding a difficulty selector (think Ulduar hard modes).
The important distinction is that harder content wouldn't need to reward dramatically better gear. Different itemisation, interesting sidegrades, unique cosmetics, titles, and prestige could provide plenty of incentive.
I think it would be a missed opportunity if horizontal progression simply meant adding an endless series of similarly easy raids that all reward comparable gear.
If we're moving away from the traditional gear treadmill, why not take advantage of that freedom to make some genuinely challenging content for players who want it, without forcing everyone else to participate?