Seems to me that all of their young guys are up and coming position players. Yes you are correct that none of them have really proven themselves as of yet so the quality is definitely questionable but I'm pretty sure that many of them with a couple of years of experience at the MLB level will become pretty good ball players
Walker, Lewis, Lee, Julien, Miranda, Rodriguez, Keashal, Martin, Larnach, Wallner, Jeffers, Buxton, are some of the guys that come to mind who the Twins have developed or are developing. None of them other than really Buxton maybe even Jeffers would be considered a valuable piece for trading and such, but most of those guys have proven that for at least short periods of time can play baseball at a high enough level that maybe they'll one day be starting quality players.
Pitching wise they have Festa and Zebby coming up along with Ober who they've developed and you could include SWR since he's young even though they got him in a trade, the Twins probably get most of the credit for developing him. So they have guys, but not the big name guys that an organization would like to have.
Correa, Castro, Vazquez, Paddock, Ryan, Lopez to me would be examples of guys the Twins have that they really didn't develop. So I think they have some guys, but not really guys that they can get big trade pieces for, at least not yet. So they have some work to do in order to have a lot of depth to be able to trade pieces away. Hopefully they don't trade Lopez away. They might be stuck with mostly what they have, have to fire sale Paddock and maybe Vasquez in order to keep Lopez?? But I'd rather have Lopez than those two.