Assuming the Shadow Lairs are essentially stratum 7 but not part of 'Tier 4', then this suggests that Tier 3 actually has a third stratum that isn't used much.
Combine this with the observation that Radiants start drop about half way through stratum 6 - or in a 3 strata layout, half way through Tier 3. Also combine it with the observation that 4* gear seems to defy the expected response in stratum 6, which might possibly not be an error at all if the scaling system knows about stratum 7. These two anomalies make better sense in a 3 strata layout than 2.
So what's the suggestion?
For general clockworking, open up the third stratum via elevator access. This may or may not require some form of tax control if the loot payout is too much for the economy to handle though. Controlling user access may also be in order due to the nature of it not (currently) being part of the mission system.
Might be a good place to find some of them giant mobs that are currently under-utilized (giant lumbers, giant danger mission mobs etc). Theres a bunch of them out there, but they are pretty much just set pieces right now.
For the Shadow Lairs, this would be highly disruptive if it doesn't adapt. Most prominently, the entrance fee may need to be re-evaluated in order to bring them in line with arcade access. With the current Key system, you are expected to pay every time; while the argument exists that the fee is balanced by not paying for the SL gear recipes and/or selling the unique materials, the problem is here is very few people want to craft multiple copies of the same gear and therefore the potential material sink is probably less than 12 per person on average.
If the key was crafted from the material itself (much like Tortodrone access), then at the very least players are motivated to get to the end each time to replace their keys for additional runs. Some number of keys will be not self-replaced due to failing the run, so there will always be some people needing to buy them from the Depot (less or more than there is now? I can't say). And don't forget sparks, for example the unknown passage exists for the sole purpose of spark munching. Reducing the boss token drop might be in order, depends on if people think an extra 3Kcr in seals is worth the risk or not.
Not necessary, but if I had my way, keys would be per-person, not per-group.. This was an issue for Tortodrone access; people joining a party and waiting for someone else to pay the tab. Selling Depot keys as a pack of 4 and splitting every pre-existing key into 4 might work?
What does any of this achieve?
More Radiants, or at least the illusion of it depending on the time-drop ratio.
Better separation between 4* and 5* gear. And who knows, maybe defense might be worth something for a change.
Inflation. Or does it? Again depends on the time-crown drop ratio. If you still have to slog through stratum 5 each time then maybe not?
Less shadow key purchases.. or possibly more? If accessing the Shadow Lair was a much more common activity, replacement keys from failed attempts might at least it might at least break even against the current uncommon "I only go if I need to craft or if someone else is paying" run?
Spark consumption.
Bringing the Shadow Lair back into the game, even including the ability to play alone without being beaten over the head with a 4 player party-centric cost.
Freeing giant mobs from the confines of their native scripted events.