More specifically, I'm suggesting that Paul served as the inspiration, as the model for the fictive figure of Mark's leper.
I'm convinced that the author of GMark used Paul's letters in crafting his tale, and there is a significant amount of scholarship in support of that concept. I would even take it a bit further, that the author of GMark was a Paulinist insider.
But why would a follower of Paul present Paul as a leper of all things? Well, first of all in 1 Corinthians 15:8-9, Paul himself self-identified with the leprous ektroma from the story in Numbers 12 because he persecuted the assembly of God, as I have characterized here --- viewtopic.php?f=3&t=2396. And like the leprous Miriam in Numbers 12, Paul was cleansed by the grace of God --- as was Mark's leper cleansed by the grace of Jesus.
One might wonder how Paul could claim to have been chosen from the womb (Gal. 1:15) and also self-identify as an ektroma, as a miscarriage, as an abortion? Paul derived his story of having been chosen from the womb from Jeremiah 1:4-5 (LXX). And just like Jeremiah, it was later in life when Paul was called by the grace of God to work among the Gentiles (Jeremiah 1:6-10, LXX; Gal. 1:15-16). More on this following the table below.
Casual readers of Mark's story in verses 1:40-45 would be very unlikely to make the connection of Mark's leper with Paul. So again, why do it? Mark was an exceedingly clever writer with a sly sense of humor. I think it was a respectful inside joke that Mark found amusing. It would not be the only instance of sly characterizations in Mark's tale.
One very significant implication of this solution for Mark's leper is that 1 Corinthians 15:8-9 --- in which Paul self-identifies as the ektorma --- as well as the wider passage of 15:3-9, was part of the letter when Mark wrote his tale. This would stand as evidence for 1 Corinthians 15:3-9 as original to the letter, as opposed to a later interpolation as suggested by a few.
The three-way table below may be easier to grasp if the process is briefly described. Paul first used Numbers 12 and Jeremiah to help craft his backstory as described in the link above. Then, the author of GMark --- as an insider knowing that Paul used Numbers 12 and Jeremiah --- used Paul's backstory and Numbers 12 and Jeremiah to craft his tale of the healing of the leper, with Paul as the model for the leper.
Paul saw himself (1 Cor 15:8-9) as Miriam the leprous ektroma in Numbers 12 who was ignorant and sinned, because Paul was ignorant and sinned when he persecuted the assembly of God (Gal. 1:13-14). But then God revealed his son to Paul and called him by His grace to preach Jesus among the Gentiles. Paul did not consult with anyone, but went to Arabia (the land of Moses), and like Miriam Paul returned cleansed. (Gal. 1:15-17).
Note: color highlights are intended to be applicable only within each row ---
|Mark 1:40-45||Numbers 12 (LXX)||Paul|
|“And Mariam and Aaron spoke against Moses” (12:1)|
“And Aaron said to Moses, ‘I beg you, sir, do not lay extra sin upon us, because we were ignorant in that we sinned” (12:11)
|“I persecuted the assembly of God to an extreme degree and tried to destroy it.” (Gal 1:13)|
[Like Miriam, Paul was ignorant and he sinned]
|“And a leper came to him [Jesus], calling out to him and kneeling down and saying to him, ‘if you are willing, you are able to cleanse me.’ And having been moved with anger ** , having stretched out his hand” (1:40-41)||“The anger of the Lord’s wrath was against them, and … Mariam was leprous like snow” (12:9-10)|
“And Moses cried out to the Lord, saying, ‘O God, I beg you, heal her!” (12:13)
|“Do not let her be like unto death, like an ektroma (έκτρωμα) coming out of a mother’s womb” (12:12)||“Last of all, as the ektroma (ἐκτρώματι) he was seen by me also ... because I persecuted the Assembly of God ” (1 Cor 15:8-9)|
|“he touched him and … immediately the leprosy departed from him and he was cleansed. And having admonished him, he immediately sent him away” (1:41-43)||“And the Lord said to Moses, ‘Let her be separated for seven days outside the camp, and afterwards she shall enter.’ And Mariam was kept apart outside the camp for seven days, and the people did not set out until Mariam was cleansed” (12:14-15)||“But when God ... having called me by His grace, was pleased to reveal His Son in me so that I might preach Him among the Gentiles” (Gal 1:15-16)|
“I went away into Arabia and returned again” (Gal 1:17)
[Paul seperated himself in the land of Moses, and returned cleansed]
|“And he said to him, ‘See that you speak nothing to anyone’ (1:44)||“I did not immediately consult with flesh and blood” (Gal 1:16)|
|“but go, show yourself to the priest and offer for your cleansing what Moses commanded^^^, for a testimony (μαρτύριον) to them.” (1:44)||“I went up again to Jerusalem … and set before them the gospel that I proclaim ... before those of reputation” (Gal 2:1-2)|
|“having gone out, he began to proclaim freely and to spread the word.” (1:45)||Sound like Paul after his calling and cleansing by the garce of God?|
There is more to the story. As demonstrated in the link and in the text above the table, Paul's backstory of his having been chosen from the womb and his later calling by God to work among the Gentiles was derived from Jeremiah 1:4-10 (along with Isaiah 49:4-6). Paul was chosen from the womb, but became like the leprous ektroma in Numbers 12 because, like Miriam, Paul was also ignorant and he sinned when he persecuted the assembly of God. And Paul, like Jeremiah, was chosen by God from the womb, but only later in life was called by God to work among the Gentiles. The author of GMark, well aware of these connections, also incorporated Jeremiah into his tale of the cleansing of the leper.
Paul was "cleansed" from his self-imposed leprous state as the ektroma when he was called by the grace of God to work among the Gentiles (Gal. 1:15-16) ---
In GMark, the leper (as a parody on Paul) was cleansed when Jesus reached out his hand and touched him ---
In similar fashion, and as source material for both Paul and Mark, when Jeremiah was called by the grace of God to work among the Gentiles, God reached out his hand and touched him ---
I think it's a serious mistake to underestimate Paul's creative and generative use of the scriptures, and Mark's use and knowledge of Paul, and Mark's use of the scriptures, and Mark's cleverness.
Are all these inter-connections just coincidences? Not likely.
Note: I believe that Mark had an additional, theological agenda related to authority in this passage about the leper.
** The majority of MSS have the term “with compassion” rather than “with anger”. But a few important MSS have “with anger” and I favor that reading as original based primarily on this analysis with Numbers 12:9 as part of Mark's source material.
^^^ Leviticus 13 and 14
ETA: I have done some significant editing and additions to this OP in January 2020.