Weirdly, I think it would be possible to use 3e's multiclassing mechanic safely as long as you changed classes so that levels gave you about the same amount every level and were all linear.
One benefit there is you would be reducing what you get with a level by a lot, so early levels would have you be pretty ineffective; way easier to play your farmers forced into heroism, caravan guards stepping up from earning their paycheck, barmaids with a sturdy iron, etc.
One downside there is that, well, early levels being effective makes people think they suck. As easy as it is to start higher level, people are bad at it. It does work in systems like Mutants and Masterminds, though. If you tell people "Heroes start at level 6, so that's where we expect default games to start. Commoners start at level 1, so that's a good spot for grittier games or for learning the system. If just learning the system, consider leveling up very rapidly."
They're definitely not doing that though - see quadratic casters, frontloaded classes... and I can't imagine them coming close to doing that, so... I'm a little baffled about what the plan is here, but hey. The road is interesting.