There are LOTS of excellent apps that could leverage Hive and thrive (in terms of usage). What is FAR more difficult is to develop an app that is ECONOMICALLY sustainable (at least break-even, if not profitable).
At HiveFest, I heard that even our top 20 witnesses cannot live off of being a Hive Top 20 witness - they would simply have to sell too much Hive and that would be self-defeating at this point, because there is not enough DEMAND, not enough BUYING pressure.
In the past people were buying HIVE because they were believing a narrative about a growing community and trusting that money-men would find a way to monetize that community somehow, which would make the coin "moon". That narrative lost its luster a while ago and nobody is spinning Hive narratives anymore.
Given the fact that Hive is not making enough people dream any longer, each app needs to stand on its own feet. So yeah, you could host the code on the blockchain, but would enough people BUY HIVE to access such a service? That's what it is about. The app / service in itself makes a lot of sense, but can anyone SELL it? And yes, targeting Reddit and or Quora, Medium, Substack would also make a lot of sense. But can anyone use such functionalities to SELL HIVE? Because in the end this is what will reverse the fortunes of this chain and of this community.
It's not "what app / service could be built better on Hive", it's "what app / service would give people a reason to BUY HIVE"
What we are building with Innopay (thanks to 
This is the "App 3" in your list above. Let's join forces and develop an Innopay / Distriator / spendHBD network of shops all over the world. United we stand, divided we fall
RE: Killer Apps for Hive (and why not)