One possible way to pull off the reveal. Introduce a new hero, in a secondary role in one of the future movies, perhaps similar to Black Widow's role in Iron Man 2 or the Falcon's in The Winter Soldier. Acting as ally to the main character but ultimately not being overly essential to that particular film's plot. It could be Citizen V but changing the name and some of some of the outward details of the character might at least protect the reveal for the general audience, though probably not the more serious fans. Or they could introduce him as someone different, perhaps a double of a character known from the comics who hasn't shown up in the film universe yet and now never will, sorry, because we killed him off screen, took his costume and equipment and dropped his naked body off in a dumpster somewhere. Play him straight as a hero, throughout the film but probably drop a scene establishing Nick Fury, knows something about this guys story doesn't add up, because Nick always knows. Throw in another unconnected scene, where one of the characters talks about a previous run in with one most dangerous threats out there, a scabbed faced megalomaniac by the name of Zemo.
Cut to post credits scene. He's speaking to a character who will later be identified as the Fixer. Before them is a large computer monitor, in which they are scrolling through the profiles of a number of different villains at large, some who've appeared before, some of whom will only be known to those who read the comics, presumably being targeted for take down. The way he is speaking however seems very different from the character we thought we knew from earlier in the film. The speech he is giving which begins lamenting on the nature of heroism and what one must do to change the world, takes on a more fascistic context as it turns to the gullibility of the general public in its pathetic need to turn to 'heroes' in its desperation to feel protected, a weakness which will be all to easy to exploit. He is searching for recruits to aid in his plans. Pan back from the display, the mask has been removed revealing the scarred visage of Baron Zemo. Now you've got your Thunderbolts film all set up. In theaters in one year.
Of course it would cool to have the Masters of Evil first but given with these films, we're only going to get to spend roughly two hours every three years, with any particular set of characters, clearly we can't do everything.