Although I think a Young Marcus Chronicles would have to have a different tact from the Young Indy Chronicles because both Indy and Marcus are pretty close in age and Indy pretty much met everyone from the turn of the century through the First World War. Using the first World War as an example, Marcus would have been the right age to serve as a mid-level officer, say a captain or major. . . .and this is a war where Indy saw extensive action as a junior officer.
Going a little further back, I think that Marcus would have been just too young to fight in the Boer War along Winston Churchill or to have gone on a journey to the Inner Station ala Heart of Darkness (Imperiallism's heyday). However, I could see Marcus doing a great stint as a soldier-of-the-Empire ala George Orwell:
[If you haven't read 'Shooting an Elephant' I highly recommend it. I think it's one of the great works on human perception.]
Bottom line, since the Marcus Brody character is really not that much older than Indy it would be very hard to capture a different age. I personally subscribe to the Marcus as adventurer camp and I'd like to think of him out having adventures -- but those stories would likely have to draw from the same historical figures that appeared in YIC (I'd guess that Marcus was the same age or only had a few years on T.E. Lawrence) and it would be a challenge to come up with novel ideas -- without going in a different direction: for example Marcus as more cerebral (rather than swashbuckling adventurer). I could live with that but how exciting would it be?
In LC, Marcus tells Indy he watched him grow up and had never seen him so concerned about his father before.
The People's 2:
In LC, Marcus and Henry, Sr. have a recollective moment when Henry enters the tank to rescue Marcus:
"Genius of the Restoration-"
Now BRODY tugs his ears, flaps his arms and touches his head.
"-aid our own resuscitation!"
They exchange a handshake.
Seems like something old friends would do, not people from differing generations.
I can gather more evidence, but I think this should suffice. Marcus and Henry, Sr. are similar in age. Indy is much younger than Marcus (who seemingly played a father figure role to Indy in Henry's absence).
I hear you Hovitosking . . . I did a bad job of making my point, which is that since YIC starts telling tales with a very young Indiana Jones, there's not a lot of material (or historical figures left) for a Marcus in his prime. Unless if you want to assume that Marcus had similar adventures during his teens, Marcus's priime adventure years (during his twenties) significantly overlap with the years shown in the YIC, notably WWI.
I think it's an interesting concept but I prefer it strictly be related to Indy and his father, dealing with how they all met and became friends. It's the usual, I know, but the bond between each of them is so great.