IIRC, the only actual browser scripting language competitor to Javascript at the time was Microsoft's VBScript, and that probably would have won. There was never really a third option on the table, just "what Microsoft did" and "something else."
It would probably have been FutureSplash Animator, which became Macromedia Flash and then Adobe Flash. For a long time Flash was the de-facto web scripting language.
Flash is scripted with a language eerily similar to JavaScript -- so similar that I assumed it was derived directly from JavaScript. In the hypothetical world where JavaScript was lisp... wouldn't Flash use that?