What do I need to develop applications for KDE5? I've downloaded all of the frameworks.

Please be aware that there won't be a "KDE 5" - but there will be KDE Frameworks 5, Plasma Next and new versions of the same applications based on KF5 and Qt 5.

In terms of Frameworks development, please be aware that no binary or source compatibility promise exists at this stage. All you should need to develop applications on top of them however is Qt 5, CMake, CMake Extra Modules (ECM) and the relevant KDE Frameworks themselves.