The BECMI Elf class is out because it doesn't fit with the multiclassing concepts and rules of the last 3 editions. That class was a gestalt combination, i.e. practically a "full" Fighter and "full" Wizard at once, and the balancing mechanic was slower XP progression.
By the standard rules since 3e, that is not possible for a few reasons: it is not generally considered acceptable anymore that a 1st level character can be effectively 2 characters at the same time, even with the old limitations this is still considered unfair. Furthermore, the AD&D idea of different XP rates gets in the way of level-based multiclassing rules.
That doesn't mean it can't be done, because I think it would be possible to even fit 2e-style multiclassing rules with 5e, provided they are "calibrated" properly. Also, there can be other ways to recreate the "feel" of that Elf class, it has been mentioned many times that the Fighter has an Eldritch Knight subclass that it is supposed exactly to make you play a somewhat Fighter/Wizard gestalt hybrid (although not before level 3).
But the real Elf class of BECMI won't be done officially in 5e, not in exactly the same way it was in BECMI.