tbf, with enough experience and talent, some players can do well in any position, particularly midfielders. I have two such midfielders (one CA6 the other CA8) with very good experience. While they are level 62 and 70 in their natural positions, they are rated at least level 52 and 57, respectively, in
all other positions (except GK). For example...here are the positional ratings of the CA6 (MR):
At worst, this player's level is only about 13% lower than his natural one at any position! In other words,
experience works a bit like positional training already. Of course, in the above example, I still think my player is rated a bit low in one or two positions, especially DMR. After all, it seems logical that a "natural" MR would probably work better as a right wingback than as a fullback, for example.
So,
if this kind of feature ever does get developed, I think it should look something like this:1) Positional training options should be limited to other positions in a horizontal or vertical formation "line" with their natural one(s) unless they were given a proficiency to start. For example, I could only train my natural MR as right winger (AMR, DMR, or DR) or midfielder (ML or MC), but not other positions UNLESS he already had a proficiency in another position. Of course, a player with multiple natural positions would have more options.
2) Positional proficiency from training should
never exceed "accomplished". No amount of training should provide the "natural" proficiency in a new position. Also, positional training should stop once a player reaches age 30, when skills begin to decline.
3) The rate of increase in a player's proficiency in a position from training should be
very slow, around that of experience gain. A rate of one proficiency level gain per 1-2 experience stars seems fair to me, although positional training would continue even after a player reaches the max experience level, provided he is still under 30.
Until then, my biggest problem is with how playing these players out of position results in significant drops in their setpiece abilities. Playing out of position would affect many aspects of a player's performance, but not setpieces (free kicks, penalties, and corners) imho. 
