Most fullbacks(DL/DR) have been known to be utility players (Lahm, Alaba, etc.). They have been known to play as CDMs (DMC) and wingers (AMR/MR). Especially if the fullback is an offensive fullback (or is capable of playing as a wing back, which used to be the DMR/DML positions).
Now, I understand for game balance reasons we can't just take a DL/DR and place him in any position and expect him to still do well, but I do find it odd that you don't allow us to train them as CDMs (DMCs). Which, if I remember correctly, is the most common use for fullbacks, especially the ones with good long shot taking.
So what I'm asking for, is that you allow us to train DL/DRs as DMCs. It's already a common thing (David Alaba and Lahm were also used as CDMs in their careers) to do in modern football.