Nuance of 楽しみにしとくね? What nuance exactly does bring saying {} instead of the more usual ? In this context, would the use of () mean "I rejoice myself _in advance_ about it" ? Context : From a friend, referencing a diner we would have together in the evening of the same day.

The word in works to add a nuance "for a period of time."

>
>
>
>
>
>
>
>
>

These five expressions all mean "I'm excited about it", "I can't wait" or something like that. These are expressing the present feeling.

The following expressions fundamentally mean the same feeling as those above, but also express the nuance "for a period of time."

>
>
>
>
>
>
>
>
>

These five expressions all mean "I'm excited about it, and will be excited about it for a period of time (maybe, until it happens.)" or something like that.

The more accurate nuance of this kind of is something like "let it be for a period of time". So, if someone says , it means that s/he keeps the feeling and won't change the feeling for a period of time.

These expressions are sincerely used by people in many cases. But sometimes, people use one of these expressions just in order to be polite, or to put pressure on someone. I guess your friend said that sincerely.