Adding checked exceptions to Kotlin would be a major breaking change, and as such, would require very strong new arguments in favor of it. At this time, given the experience of many different languages which do and don’t include checked exceptions, and the fact that large stable systems with several tiers and multiple modules have been built in languages which do not include any form of checked exceptions, we do not expect such new arguments to ever appear.
As for optional warnings, this may be possible, but it’s not on our roadmap for Kotlin 1.1.