Are there any hypothesized indo-european languages?

What I mean is if there are any theories about non surviving indo-european languages and/or language families, for example as substrates of the surviving ones, that also have not been reconstructed from existing ones or otherwise attested (like Tocharian has) but simply hypothesized to explain for example a certain substrate, or similar.

For example, was there another indo-european group and language in Scandinavia before the proto-germanic group? What I don't mean is theories related to the present or historically known languages or language families, that exludes languages such as Thracian, Tocharian et cetera.

Any mentions or theories you have come across would be welcome!