The problem with adding more avenues of gear progression is that those activities essentially become mandatory for proper character progression. It's not going to be "You can do Nu-Eureka or raid to gear up, choose the one you like better!", it's going to be "You have to do Nu-Eureka *and* raid because you need to gear up ASAP." - that is, unless they explicitly link the loot lockout of one to the other.
Personally, part of the reason I burned out on WoW and started playing FFXIV was that Mythic+ A) felt mandatory, B) wasn't really that fun in the end (might be related to larger, overarching issues of combat design in that game though), and C) forced you to slog through content you weren't going to get worthwhile rewards from (sub-level-10 keystones) to complete (unless you had a dedicated group that always had a keystone ready). Not to mention the wild swings in difficulty depending on how the randomized dungeon selection you got on your keystone combined with the weekly affixes (and the insane RNG on the loot chests).
TL;DR: Adding additional gear progression routes risks burning out your players if you do it too much or do it wrong.