Unlike in the Roman liturgy, in Lutheran liturgy the post-communion collect is an ordinary, not technically a proper. Yet, our Lutheran Service Book does provide three of them, and I think they work best when used seasonally. Here's my suggestion:
The regular collect, used the lion's share of the time, is Dr. Luther's from the German Mass: "We give thanks to You, almighty God..." This prayer is so beautiful, and it captures exactly what we beg from God through our use of the Holy Sacrament - after we thank Him for the refreshing gift - to be strengthened in our trust in Him and in our love (though I wish the brünstige of the German came through better - BURNING love!) toward one another.
During the seasons of Advent, Christmas, and Epiphany, we use "O God the Father, the fountain and source of all goodness..." This prayer specifically references "in loving-kindness [You] sent Your only-begotten Son into the flesh." The Father's covenant faithfulness and His tender mercies lead to the gift of the Son into our flesh and we remember that it is the enfleshed Son we have just received under the consecrated bread and His holy blood in the consecrated wine.
During the weeks of Easter, we use the prayer "Gracious God, our heavenly Father..." with its reference to the marriage feast of the Lamb in His kingdom. A fitting reminder of the early church's joy in the great 50 days as a bit of heaven on earth itself - the 50 days being a foretaste of the Kingdom!
Thus, all three prayers become familiar across the year's worth of services and yet a subtle shading of the particular season also has a place. Anyone else out there use a similar schema?