Re: Nothing is nothing more that I can ask for.
There is [nothing more (that) I can ask for ___ ].
No: that" is not a relative pronoun, but an optional subordinator here, functioning as a marker. It's introducing the relative clause "(that) I can ask for", and is the same "that" which introduces declarative content clauses, cf. "I know that he is that ill".
"That" is what ESL teachers and learners the world over think of as a relative pronoun, and it is very much different, on the surface, from the "that" in "I know
that he is ill," as can be seen by the fact that we can replace "that" with "which" (which no one will deny is a relative pronoun) in the OP's sentence—in which case we can even have pied piping ("There is nothing more
which I can ask
for" / "There is nothing more
for which I can ask")—whereas we cannot replace "that" with "which" in your "I know
that he is ill":
*[strike]
I know which he is ill[/strike].
I'm aware that you can still argue that "that" is not a relative pronoun in the OP's sentence, but then you should include, as part of your argument, that "that" is NEVER a relative pronoun, and I'm afraid that you will have a very hard time convincing ESL teachers and learners of that, since it seems to function as a relative pronoun whenever it can be replaced by a wh-relative pronoun and has been thus described in non-theoretical grammars for centuries. Technically, though, I go along with your subordinator analysis (I'd call it a "complementizer," the head of the relative clause CP [complementizer phrase]).
In "that" relatives, the wh-relative pronoun moves into the Specifier of the CP and gets silenced, allowing "that" to masquerade as a relative pronoun. I fail to see why it would benefit ESL learners to be taught such things. That's why, in Post #9, I didn't call "that" a relative pronoun but allowed my readers to infer, incorrectly, that I think it is a relative pronoun. If it looks like a relative pronoun, acts like a relative pronoun (except in Pied Piping:
*[strike]
There is nothing more for that I can ask[/strike]), and has been described as one for centuries in traditional grammars, why not let ESL learners believe it is a relative pronoun in such sentences?
