Excellent question, wonderful to be asked. I'm a little late to the party, I know, but here are my thoughts:
//What are the capabilities which would help you to adopt the modern SharePoint development practices?//
Make the SPFx consistently and seamlessly "work across": across page types (modern/classic/publishing) and across On-Premises and Online. I know that FP2 is supposed to make the latter possible for web parts, but analysis of some recent changes to the yeoman generator seem to point to a separate project type for On-Premises, which would be a mistake. In a hybrid environment, no developer is going to want to write the same web part twice. For extensions, they need to work across page types -- if you have a standard footer you want on every page, you are currently out of luck if all your pages are not modern ui.
//What functionalities are missing?//
Forms. Forms, forms forms. Yes, I know, PowerApps...but really, this is not a viable solution for anything with really complex requirements (great for citizen developers, though). We need something in the framework that is TS/JS/HTML/CSS that allows us to quickly and simply save whole pages of entered data as separate fields or alternatively as a complete JSON object in a single list field.
//What kind of documentation or guidance would be needed?//
The documentation, while better than even just a few weeks ago, is still woefully inadequate for a beginner. The "description" column needs to be filled in for each entry. And examples that are *meaningful* need to be added. We need to know not just "what" is in there, we need to be shown "how" we should use it and most importantly, "when" and "why" we would want to. I know that this stuff is hard and a big expense to the company, but really it should be considered part of the cost of doing business. Updating the docs to match new changes should be part of the release process -- if the docs aren't ready to be updated, the software is not ready to release! This is particularly important now that changes come so frequently. No one should have to go out to trawl the internet for posts on how to do basic stuff, because things change quickly and those posts become outdated. If the docs are kept up to date with the software, this problem goes away. Additionally, some focus on past vs. future for long-time SP Dev's would be welcome; something like "if you used to do this <C# code>, now you would do this other thing instead <TS code>".
Thanks for listening!
