Jdt-LS relies on JDT for most features.
WTP and JBoss Tools do extend JDT to provide additional features, such as Jax-RS completion.
It would be worth investigating whether just adding a few bundles from wtp and JBoss Tools into JDT-LS can turn JDT-LS into a powerful tool for Java EE as well, and to evaluate what must and can be improved to enable it.
As a good Java EE support requires a good Java support first, it's very likely that extending JDT-LS (on demand) for Java EE is the most productive and efficient way to have a Java EE language-server.
As Jakarta EE moves to Eclipse.org, there are some discussions to improve JEE tools in Eclipse IDE and in general. So building on top of WebTools/JBoss Tools and allowing webtools/jbosstools to run in JDT-LS wo uld fit in the community mindset and may attract 3rd-party contributors to Eclipse Webtools.