Sacrificed Lamb By Ibrahim عليه السلام
That was not a new Ibadah, Animal Sacrifice has had been in practice as a symbolic expression of sacrifice of one’s life for God since the times of Adam (AS).
The interpretation of this dream was that he (Prophet Abraham SWS) was required to devote his son for the service of God’s House. Annual animal sacrifice tradition was a corollary of this dream you can say. Prophet Muhammad (sws) also acted on this dream (48:27) and later on it resulted in Hudaibiya Treaty. Yes it was a glad-tiding also but immediate purpose has been explained in Quran itself. Prophet (sws) and Sahaba (rta) when left for mecca to perform Umra on the basis of this dream shown to Prophet Muhammad (sws), had no clue what will happen there because they were certain that they will perform Umra right then. Similarly, purpose of dream shown in 08:43 is not glad-tiding as per Quran, since Quran makes it very clear that it was shown so that Muslims stay motivated and steadfast to face their enemy.