I've been thinking about this a lot lately, and I think that the way forward to get the "feel" of the old 2nd ed multiclassing is careful multiclassing 5e by doubling down on the subclasses. As you say, a level 5 fighter/five wizard is... underwhelming.
What you might want to try is to multiclass unevenly but compensate. For example, you could make a level 7 fighter/3 wizard but have the 7 levels of fighter be eldritch knight, and the 3 levels of wizards be something very useful to you - like a diviner or perhaps an abjurer? (an interesting alternative would be 3 levels of hexblade with the Tome pact). This would be a very strong "gish" build.
Alternatively, you would have a level 7 wizard level 3 fighter. In this case to compensate for the only 3 levels of fighter, makes the 7 levels of wizard a bladesigner, and the 3 levels of fighter be battlemaster. This would also be a vey interesting and strong build.
Fighter mage thief could be achieved by combining arcane trickster with bladesigner. I've seen it done once in a PBP game and the character was super effective.