The Torah says Rosh Hashanah is a יום תרועה, a day of shofar blasts, and זכרון תרועה, a remembrance of shofar blasts. This seems like a contradiction.
The Gemara (Rosh Hashanah 29b) suggests that the remembrance of shofar blasts refers to Rosh Hashanah that falls on Shabbos. The Gemara infers that this would be a Torah prohibition. There are questions on this suggestion, and it is subsequently rejected. Blowing the shofar on Shabbos RH is merely a rabbinic prohibition.
What was the hava amina, initial understanding, that it would be a Torah prohibition? Why would it be forbidden? What kind of prohibition would it have been? The Gemara states it's not a Melacha. Would it have been a new Melacha? Or a side prohibition, like having your animal rest?
I'm asking not just from the Hava Amina but from the Yerushalmi which makes the same suggestion as above to resolve the verse, and makes no mention of a rabbinic prohibition. The Damesek Eliezer (see below) says there's no mention in the entire Yerushalmi of a rabbinic prohibition. I heard the same fact in the name of Rav Meir Simcha. It sounds like the Yerushalmi sees it as a Torah prohibition.
I'm also wondering about the Biur HaGra. The Shulchan Aruch (OC 582:6), following the above Gemara that even though it's a rabbinic prohibition, the verse implies that there is no shofar on Shabbos, says to mention זכרון תרועה in the prayers. The Biur HaGra when writing the source of this halacha, says:
רא"ש סוף ר"ה וגמרא שם כט ב' ועי' בירושלמי שם
Rosh at the end of Rosh Hashanah, and the Gemara there 29b. And look at the Yerushalmi there.
Why is the Gra pointing to the Yerushalmi? He's letting you know it holds its a Torah prohibition? He holds its a Torah prohibition? Something else? Why mention it in a halacha about prayers. I looked at the Damesek Eliezer there and he wasn't so clear to me. Subsequently I saw Rav Shmuel Bornstein asks this in his sefer Shai LaMoreh.
If this last question isn't connected enough to the first one I can make it a separate question