Sluices where the wh- phrase is followed by else are different. Maybe more different on some theories…
Barros and Kotek (2019) (Ellipsis licensing and redundancy reduction: A focus-based approach) do not consider wh- else sluices, like:
- I'm already prepared to play Chopin nocturne op 9 no 2 but don't know what else.
How would this case fit into their picture of focus-based licensing, where the union over the focus semantic value of the sluice clause has to equal the union over the focus semantic value of the antecedent?