If the following image is anything to go by, Gabriel Luna might be suiting up as Ghost Rider for the untitled Avengers 4 movie currently filming.
Gabriel Luna, who debuted last season on Marvel's Agents of SHIELD as Marvel TV's Spirit of Vengeance, posted the following image of himself sitting next to Frank Grillo on an airplane.
It's unknown if the image is of Frank Grillo heading to Atlanta for Avengers 4, coming back from Atlanta, or possibly going somewhere else, but it's pretty interesting Gabriel Luna is on board, and also that the two are apparently seated together.
Regarding the possibility of Ghost Rider actually appearing in Avengers 4, if you would have asked me a few months ago, I would have responded with a resounding no.
However, things may have changed as it does almost appear as if changes are happening internally within Marvel.
Long story short: Marvel Studios and Marvel TV are two separate companies at Disney and said to not get along due to Marvel Studios president Kevin Feige's ongoing feud with Marvel CEO/Chairman Ike Perlmutter. This is why you never see the Marvel movies ever recognize anything Marvel TV has ever done, and this is why the Marvel TV characters never have appeared in the Marvel films. Feige booted Perlmutter and his Marvel Comics and TV cronies from being further involved with the movies due to reportedly causing all kinds of interference and problems (which is why Joss Whedon left Marvel, etc.).
It's possible things may be changing within Marvel due to a variety of factors, but recently Brian Bendis bailed on Marvel to go work for DC Comics, with a reason cited because he is no longer involved with the movies (the Marvel Comics Creative Committee). Is it possible the Marvel Comics Creative Committee is no more? Is it possible Ike Perlmutter is out or stepping down from Marvel? Perlmutter has since become involved with Donald Trump's presidency and has even been said to have dropped the Marvel CEO title and is now known as the Marvel Chairman due to taking on less responsibility (Perlmutter is 74-years-old as well). So perhaps Perlmutter is eventually going to take leave from Marvel, which would mean someone else would take over? Maybe someone Feige is on good terms with?
Something else to consider is that Marvel TV (under Perlmutter) has seen a string of poor quality shows, which includes the recent IMAX-funded Inhumans, and Marvel TV can't even get their own network (Disney-owned ABC) to pick up their New Warriors comedy series. Another potential factor changes may be happening within Marvel is that with Disney launching their upcoming new streaming service to compete with Netflix, Disney may not want the poor quality Marvel TV has been offering as of late, as the streaming service will feature a new Marvel project.
So maybe, just maybe, Disney has decided to have all of Marvel under one umbrella, instead of divided ("a house divided.." eh?).
Further proof that something is potentially happening within Marvel is that Samuel L. Jackson and apparently SHIELD is returning to the Marvel movies. Feige killed off SHIELD in Captain America: The Winter Soldier and Nick Fury hasn't been seen since Avengers 2, but Jackson returns for Captain Marvel, and it's reported SHIELD appears in Ant-Man and the Wasp.
Two more factors to consider are the recent talks between Disney and Fox, as Disney wants to purchase Fox Studios (and the X-Men, Deadpool and Fantastic Four), and Marvel Comics has recently changed its direction and is now returning to focuse on original characters (another reason why Bendis is said to have left, and it's possible a new Marvel Comics EIC could also be announced in the near future).
To be clear this is all guess work on my part, and probably a majority of the fans aren't aware of the Marvel Studios and Marvel TV disconnect, but possibly Avengers 4 might feature the TV characters, which would give fans what they have literally been begging years for: some sort of Marvel "TV" and movie crossover.
Even if Feige showed a few of the Marvel TV characters briefly in a cameo, that would go a long way to show how supportive Disney and Marvel are of the fans.
So if we see Gabriel Luna transforming into Ghost Rider ready to avenge the fallen Avengers in Avengers 4 - look for the Spirit of Vengeance to take on Thanos(?!) - meaning things may be changing within Marvel!