Some languages in the past have been influential without ever reaching wide adoption. For example, many languages owe much to the design of Algol 68, even though few compilers were ever written for it. The Dylan language was killed by Apple but had a clean and interesting design.
What other programming languages had cool ideas but-- for whatever reasons-- didn't make it to the mainstream? Is there an interesting language feature that you wish your main language had? Is there a feature ahead of its time that we'll soon see used?