Extending Eclipse and IntelliJ IDEA
Possibly, you have some ideas of your own? You can then turn to Aivar with Eclipse topics and Vesal for IntelliJ. If you find the concept of having your own idea too old-fashioned, we could suggest a few things.
IntelliJ Plug-in for JML
I really like the KeY project, except all their tools are using Eclipse, which I do not like. If you are interested in IntelliJ platform technology and would like to make a nice contribution to an open source project, creating IntelliJ Plug-ins for these KeY tools would be a great idea. Initially, all we need is support for OpenJML to IntelliJ IDEA.
Support for Cofoja in the IDE
Contracts for Java, Cofoja, is a more lightweight verification framework than JML or KeY. It simply allows one to write pre- and post-conditions as Java annotations. These are checked at runtime. This is very much like OpenJML, but the use of annotations means that it can be implemented as a Java library and this is more modern. The small problem here, though, is that annotations aren't syntactically checked by the IDE and are only displayed as plain strings. It would be awesome to have the typical IDE support for assertions written in these annotations.