I have a question for advocates of 3e/5e style "Dip" multiclassing: If you can play a fighter 1 / mage 6, what is the point of having the classes at all? I mean, I understand wanting to play a fighter / mage as a character concept, but a fighter 1 / mage 6 isn't a concept...it's an optimizing tool. At least my limited exposure to 3e saw it used that way. Do people who are in favor of 3e/5e style multiclassing like it precisely because it lets them optimize?
I don't, I am in favor because (as I mentioned previously) it enables narrative concepts such as "character who picks up a secondary area of expertise" and "character who changes his path in mid-life" which are not possible with AD&D-style multiclassing (dual-classing could cover the latter, but definitely the old rules for this weren't working fine). For the former purpose, which IMO is more important, 3e-style multiclassing gives you all the freedom you want.
As much as I don't like the game of optimization through combining classes, it is a legitimate playstyle in D&D, and incidentally it has made WotC a fortune in splatbooks sales in the 3e era.
The core of the problem is that such playstyle can be incredibly irritating for other players at the game table. How are we going to solve the table incompatibility between the optimization-lovers and the optimization-haters? By cutting off either half of the gamerbase from the next edition? You can forget about that, WotC would never willingly cut off so many potential customers! And what if they decided to cut off your side instead, because they think it's slightly smaller?
Multiclassing only allows you at most 2 classes, and no class can be more than 2 levels behind the other.
IMHO the best they can do is just label multiclassing as "OPTIONAL". Just that small label is enough to tell any optimizer that the optimization tool which is multiclassing is not something they are granted by divine right, but rather something that needs to be
enabled by each gaming group depending on their overall agreement, knowing how it can change the game.
I've played 3e games where one of the main house rule was "no multiclassing", and it's always easy to do so, but an official label can help a lot, because all new DMs are probably just going to assume that everything that is not specifically label as optional is always fine and has no problem.
A simple sidebar or small paragraph can suggest optional restrictions like "max N classes" (which works great) or "max L level difference" (which doesn't work so great). But again they should better be suggestions to the DM rather than hard-coded rules.
The DM and players should get the feeling that they are free to choose how their game should work, not feel like they are being patronized about how they should play, but instead feel like they are being informed about how they could play the game in different ways.
Thus that sidebar/paragraph could explain briefly to the new DM what happens to the game if multiclassing is freely allowed, and tell her to consider maybe to run her first game with some restrictions (e.g. max 2 classes per PC) and see how it works, before allowing full freedom.