The sad thing, for me, about IEs is that they had the bones of an excellent idea which was badly handled. In their purest form they could be put into any zone in the game and adapted to that expansion to be a fun group and solo feature.
How would I do that you ask?
I am happy to tell you. First, take out the timer. It was one of the worst idea they had for IEs and was generally why it failed as a game activity. Timers might work (barely, imo) on M+ but M+ gives gear. IEs did not. So timers were guaranteed to have them work as speed runs for no real gain.
Second, remove the game NPC-other faction from the normal runs and only have player-v-player for one of the options. Without the timer competing with you for azerite, there is no need for the opposite faction npcs.
Third, have a quest for each IE that requires you to locate something (either an item like a diamond-encrusted banana, or a mini-boss through a portal or deep inside a cave) that will reward you with some form of goody and the quest loot.
Fourth, have currency (similar to the dubloons of BfA) drop from mobs which can be used to purchase consumables + a thing I’ll call a Luckydoo. You can have one of these at a time and using them will increase the drop chance of mounts and pets by, say, 50% for the run.
Fifth, have solo entry versions as well as groups, up to 5. No tank/healer/dps required, just 5 players. And it scales depending on how many are in the group, as happens in Torghast.
They could design the IEs to match the environment and story of each expansion they were in. Icy islands with Undead for Wrath, desert islands with Draenor nasties, etc.
I truly believe this was something Blizzard dropped the ball on. It could have been a great addition to the game for everyone. Its kind of sad that they came up with a good idea and just blew it.