One simple change that would make the life of a lot of developers on Steem a lot easier:
Add an app property to every post and make it part of the API
It is really hard for apps on Steem to separate the posts from other apps. Some apps maybe don't won't to display posts from other apps or they don't want that they show up on Steemit.
Tags alone are not enough. Every user can set that tag on Steemit. There has to be another property for apps. Every app already sets an app property in the metadata of every post. Make it part of the post, make it mandatory and make it part of the API.
Another important thing in my opinion is:
Focus your API for Steem apps and not only for Steemit
Almost all API calls are for features on Steemit. The API has to be more general. There are a lot of queries that are important for other apps on Steem that aren't possible at the moment.
And the most important thing:
Document your API
I wouldn't call that documentation what you are showing on https://developers.steem.io/. That are just all possible function calls, nothing else. There is no parameter description at all. There is no documentation how to calculate voting power, Steem Power and a lot of other things that Steem app developers need.
It is really hard to get started as a developer, because of the lack in documentation.
The only real documentation is the source code or user guides.
Appbase has many API improvements. One of them is a new database_api that is de-coupled from Condenser (steemit.com). It allows for paginated query of all database objects by all orderings the blockchain uses.
It may not be all of the information you need, but it is substantially more than what is currently available.