Add missing rows with missing date and time in Power query

Filling down should come after replacing null with something else so the replacement is included in the filldown as well.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("fdBNCoAgEAXgq4hrwxnHH2jXAYL24qLaWq26f0Vhgel2+HiPN97zcZobAEQuuJUoFSgtGKoWgHX9eUTgQZRYUuseY9m9aVRJo6T0j6KvGi5lfpR7lE7K3mpb1gZrKyssKVNGb5QrI6r25QspR/nA61nhAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, #"DateTime Value" = _t, Value = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"DateTime Value", type datetime}, {"Value", Int64.Type}}, "en-US"),
    #"Replaced Value" = Table.ReplaceValue(#"Changed Type",null,"this is null",Replacer.ReplaceValue,{"Value"}),
    #"Grouped Rows" = Table.Group(#"Replaced Value", {"ID"}, {{"MinDateTime", each List.Min([DateTime Value]), type nullable datetime}, {"MaxDateTime", each List.Max([DateTime Value]), type nullable datetime}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "DateTime Value", each List.DateTimes([MinDateTime], Number.Round( (Number.From([MaxDateTime])-Number.From([MinDateTime])) * 24, 0) + 1,  #duration(0,1,0,0))),
    #"Expanded DateTime Value" = Table.ExpandListColumn(#"Added Custom", "DateTime Value"),
    #"Removed Columns" = Table.RemoveColumns(#"Expanded DateTime Value",{"MinDateTime", "MaxDateTime"}),
    #"Changed Type1" = Table.TransformColumnTypes(#"Removed Columns",{{"DateTime Value", type datetime}}),
    #"Combined Original and Expanded Dates" = Table.Combine({#"Replaced Value", #"Changed Type1"}),
    #"Changed Type2" = Table.TransformColumnTypes(#"Combined Original and Expanded Dates",{{"Value", type text}}),
    #"Sorted Rows" = Table.Sort(#"Changed Type2",{{"ID", Order.Ascending}, {"DateTime Value", Order.Ascending}, {"Value", Order.Descending}}),
    #"Applied Table.Buffer" = Table.Buffer(#"Sorted Rows"),
    #"Removed Duplicates" = Table.Distinct(#"Applied Table.Buffer", {"ID","DateTime Value"}),
    #"Filled Down" = Table.FillDown(#"Removed Duplicates",{"Value"}),
    #"Replaced Value1" = Table.ReplaceValue(#"Filled Down","this is null",null,Replacer.ReplaceValue,{"Value"}),
    #"Changed Type3" = Table.TransformColumnTypes(#"Replaced Value1",{{"Value", Int64.Type}})
in
    #"Changed Type3"

danextian  Thank you very much, it worked:)

Glad it did. Could you please accept my pose as solution?