This is asked for on every forum development that I belong to... ie... IP.Board, Burning Board, XenForo, phpBB, SMF, vBulletin, FluxBB, ect.... And no matter how many bones someone toss out, people always claim they want a detailed roadmap.
I'm guessing what they're expecting is
1.2.4 - add x, subtract y
1.2.5 - optimize z, add 123
1.2.6 - add 456, improve 789
That's never going to happen on any development. Doesn't matter if it's IPB, Burning Board, XenForo, phpBB, SMF, vBulletin, FluxBB, ect....
Why? Competition. If you tipped your hat, the other guy is going to rush to release X, Y, Z, 123 before you do. Then when you actually release x, y, z, 123 they'll claim that you're copying them.
I use to have such a roadmap on my site. First thing my competitor did was push it out before me. Even down to the some of the early screen shot and descriptions.
And in the case of a paid development, then you've X corporation ready the sue the hell out of you the moment you release x, y, z, 123, because they had it first, even though you planned it first, but when has that stopped a large corporation from being silly? And if they don't get sue happy... At the very least they can say, "hey, we had that first & the other guy is just a rip" Which makes one hell of a selling tactic.
XenForo does release a few helpful and insightful breadcrumbs before they release things. As do other developments. So people do get a few cookies and bones tossed in their direction and are kept as informed as possible.