The issue is that M+ was very enjoyable for many players since introduced back in Legion but without the Vault allowing it to drop gear comparable to Mythic raiding at some point your M+ progression gets capped unless you do Mythic raiding as well.
Imo the better solution would be to keep the cap to the end of dungeon rewards, remove the ilvl bonus for Vault gear but also remove the cap to ilvl scaling for Vault rewards; so your Vault would drop heroic ilvl for a +15 but if you did a +25 then the end of dungeon loot would be heroic ilvl but the Vault reward would be Mythic ilvl. BiS would still require raiding because of end of raid bosses dropping higher ilvl loot though. Alternatively just expand the Valor system to let you upgrade gear to Mythic ilvl for a sufficiently high Mythic rating, counting on the Valor cap to keep gains under control.
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I'd really love if raids had more mini-boss type trash. Many raids these days have those, but I'd love to see it expanded and have those drop rewards on a weekly lockout (one lockout for all difficulties); with professions around those rewards could be rare reagents. Make trash somewhat interesting by giving them a few more mechanics and rewarding to do.