Cake. Here you go:
Matthew 17:10 And his disciples asked him, saying, Why then say the scribes that Elias must first come?
[11] And Jesus answered and said unto them, Elias truly shall first come, and restore all things.
[12] But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them.
Not only is it prophesied that elias must return first. Not only is it noted by one of the apostles. It is confirmed by Christ who then states Elias had already returned and had been "dealt with" in a fashion that Christ would also suffer. IE, can you find anyone in Scripture that matches this - there is only one person that had to come and make way for Christ. There is but one man who suffered and died at the hands of the Romans to this point. That was John the Baptist. Mark 9:11 - 13 reiterates the above. And then there is Luke:
Luke 1:17 And he shall go before him in the spirit and power of Elias, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to make ready a people prepared for the Lord.
If there is any vaguarity about this, Jesus has cleared that up. So there is no problem with saying Elias returned in the person of John the Baptist and made the way for the ministry of Christ. Once dead, he could no longer return and influence things on earth and as moses was also dead, only a vision of them could appear in the other instance aforementioned.
Yeah, I kinda thought that's where you were headed. So let me see if I have this straight. You don't believe Elijah was just an archetype of John the Baptist, you actually believe John the Baptist WAS Elijah returned to Earth.
Just to flesh this out a bit, John the Baptist is often credited as being Elizabeth's son. Was he?
I've taken the liberty of pinging a number of other Protestants to this conversation since I'd like to see if this interpretation is widespread among Protestants.