It is unlikely to have another JS engine. But the JS engine is exposed for WKWebViews, where creating a shim layer could work perfectly fine for many extensions.
The issue is not the technical capability. It has been possible to inject javascript into web views in ios since ios 6 (at the least). The problem is that Apple App Store terms state that your app cannot download arbitrary pieces of code from the internet and run it, which is exactly what extensions are. The rule was relaxed recently, to allow for things like teaching programming apps for the iPad pro, but not for browsers.