As far as the challenges go, I'm actually not even against having a mix of 'casual - medium - veteran' levels of challenges/tasks. The store also has potential, but it likewise has issues.
The way I see it, Nightwave is a failure if you see it as a replacement for alerts. Because it isn't. Gear requirements and such aside, because the wolf credits are limited, you are forced to only buy certain things. As a new player, for example, you'll need a bucketload of nitain. 180 to be exact. Even if you managed to complete the missions and such, that still means you'd have to spend (180/5)x15= 540 wolf creds (or the future equivalents) just to buy the nitain. That leaves you with practically nothing to get other things from the store.
With alerts, you missed things because it's essentially a recurring timed exclusive. With the current Nightwave store you miss things because you're forced to pick other things.
Now, the idea to replace alerts with a store where you can buy whatever from the 'alert pool' isn't a bad idea at all. It simply needs refinement so it's actually worth using instead of just being a pain in the exhaust port.
There are a few ways the primary annoyances could be circumvented, without actually making them easier to get time investment-wise.
- Bring back alerts, but in the form of currency source: The alert store would list things like it already does, preferably with all the alt helmets in stock, but a limited selection can still work. Alert series levels could be the same as they are now: a collection of various items, from somewhat common to cosmetics to highly unique items like umbral forma. The alerts themselves, however, would be with regular intervals (30m, 1h, whatever fits, as long as it's not too spread out). They would be the primary source of alert store currency but in low amounts/alert. Alert store levels would have chunks of it, as it does now.
- Make the alert store a 'contract board': In essence, you'd remove nearly all items from the alert store. Instead, you have contracts for missions with set rewards. Think of it as picking which alert you want to do. To do these contracts you'd have to purchase the details with normal credits, with costs scaling to rewards and mission level. Single use only, even if you fail it. There can even be a cooldown on a contract for, say, a forma, so you can get one from an alert, but if you want to get more, you'll have to wait. Items like nitain would have a short cooldown, depending on how many you get per contract. You could use the series level system for either unlocking higher tiers of alert contracts (more items / shorter cooldowns / lower purchase costs / etc), or just use it as a mini-event people can work towards, with missions as we have now. The rewards for that can be pretty similar to what WoS6 has: cosmetics / forma packs / unique decorations / sigils / etc.