I have recently been frustrated by not being able to create a subclass of a decompiled class using the Alt-Enter intention.
It has happened enough recently to bug me enough to post about it, but I thought I should check youtrack first.
It turns out that I raised the same issue 5 years ago!
That was closed as Fixed without actually fixing it....
There is also this post suggesting a plugin, https://intellij-support.jetbrains.com/hc/en-us/community/posts/206249639-Create-Subclass-Intention however, the plugin is too old and doesn't work anymore.