Single or multiplatform? Which OS?

Will there also be a mobile or web version of an app? Or embedded?

How well does it need to integrate with target platform? How native it must feel? Does it access to native platform features?

Does it need to be lightweight?

Does it need very low latency? Is performance critical?

Does it need many custom widgets or none?

Does it need to have quality implementation of any specific widgets available (like charts, maps etc.)? Big widgets ecosystem?

Does it need heavy widget trees?

Does UI need to be customizable by users?

How important is accessibility?

Programming language preferences and limitations?

How important is developer productivity?

Can framework be proprietary or non-free?

Is it maybe just to learn UI development?