It seems as there is some concern over the comma that was written into this verse. How do we know it was written in? When we speak, there are no commas, periods, or exclamation points. We have to listen closely and we discern where a comma or a semicolon might be put in when we write down another person's words. We add punctuation points that might be put in when we write down another person's words. We add periods, commas, exclamation points, etc., to help us as readers when we are reading. To me, adding the comma in the verse is irrelevant as to the meaning of the verse.
The author of this gospel was not present at the cross when Jesus made this statement. The verse was written down from people who heard what Christ said at the cross. So the verse was written to convey the spoken emphasis of the statement.
The verses before verse 43 give us some sense of what is to come in this verse. As the two thieves and Jesus were on the cross, they were having a continuous dialogue (elegy Greek). The first thief in verse 39 is attacking Jesus and His deity. The second thief was dialoging with Jesus over a period of time. This second thief was sincere and had a real request of Jesus even though hanging on the cross. One criminal's attitude produced blasphemy, ending in the statement, "If you are the Christ save Yourself and us." The other criminal gave a rebuke of the first criminal's wrong attitude and ended up with a statement to Jesus, "Lord remember me when You come in glory and power." This is the correct reading of verse 40, not, "..when You come into Your kingdom." The Greek en makes the verse read, "..when You come in Your kingdom." The second thief's request was to be remembered not on that day but at the time when Jesus returns to power and glory.
When Jesus replies in verse 43, He starts with "amen", which announces that a Truth is about to be revealed. Putting today at the start of the next phrase is telling us that this is the truth that Jesus has promised. In effect, Jesus is saying, "It is not a case of remembering you, the second thief. You don't have to wait for a future time! You will be with Me, as of today."
The same term for Paradise is used in other parts of the Bible. The same term is the Garden of Eden; Gen 2:15 and 3:23. The term is used for the third heaven; 2 Corinth. 12:2-4. Also, the term is used for God's Kingdom; Rev 2:7. Is Paradise a place where righteous people go after they die and before they await their resurrection?
The thief on the cross recognized that the Man dying next to him was the Messiah and repented. Jesus heard the cry of the thief and even in dying, proclaimed that he would be remembered when Jesus returns in power and glory. Jesus, in essence said, "Your request was as good as done now." Jesus assured the repentant thief the truth that even on that very day; he would be counted among the righteous in Paradise.