Yeah Ben Browder could easily have played a Tony Stark-alike character. Hell I could have seen him as Abe Jenkins aka the Beetle aka Mach IV (or are we up to Mach V now?) from Thunderbolts. Or even hell a more snarky Cyclops. Or heck... Nick Fury. (if we're dealing with 616 but since we're not really...) maybe even Norman Osborn he could do the insane laugh. LOL Or... heh an older Peter Parker. Sure BB is a bit bigger than Spidey in the movies has been, but who says Spider-Man HAS to remain a kid? He's got the snark and attitude. The quips during fights work with him. Or hell Iron Fist.
There's a number of Marvel roles he could have played, but Captain America ain't really one of them.
What they really need to do is find a relative unknown for the role, someone that can be known as Captain America, that is young enough to do it for a while. Someone who could even play the character on any live action TV shows too. I know RDJ is too busy to do a lot of extra work, if they ever did do a made for TV thing or an actual Marvel based series, where some of the Avengers pop up every so often, but all they need to do is have some other guy in the suit and just dub in RDJ's voice. How many issues of Iron Man or the Avengers did he practically spend the entire issue in the suit. Especially during the Rhodey years
Cap should be youngish-mid20s ...maybe late 20s but no older, unless you're casting the role as oldCap and youngCap