So last I read the whole Captain Marvel trademark situation is a bit of a sore spot in the DC./Marvel relationship.
Well marvel did get the trademark fair and square, it was lapsed and thus first come first serve.
Anyway......let's try this idea. It's not quite a crossover so much as a lease or a time-share deal, similar to what Marvel and Sony did to get Spidey into the MCU.
An event occurs in DC comics, for example Dr Manhattan banishes Shazam to another universe. It is only Shazam, his support characters like Tawney the Tiger, Mary Marvel, etc all remain...but without him they lose their powers.
Billy arrives in the Marvel Universe and while he is trying to seek a way back to his dimension, he still has his powers and thus still fights the good fight on Marvel Earth.
Thus we can get some interesting stories of how he would or would not fit in with the Marvel characters, and of course he would have to fight some of them.
Imagine: Shazam vs. The Immortal Hulk (or any other persona of The Hulk)
Shazam vs Abomination
Shazam vs THOR (the REAL THOR, not the emasculated wimp of hammers that is currently in the book)
Shazam vs the Asgardian DESTROYER armor
Shazam vs Juggernaut
Dr Doom: he always wants power and knows magic, he would be very interested in Shazam's power source.....
Shazam vs Silver Surfer and Galactus!
Shazam vs Hyperion
As a further spin on things, soon after arriving in the MU, he realizes his name is supposed to be Captain Marvel....thus he starts using the name which draws the attention of Carol Danvers.....
Anyway it is an idea I had, kind of a sink or swim thing to see if Shazam COULD be used well in the MU and possibly lead to a story where he gets to choose: stay in the MU or go back to the DCU.....
His decision of course reflects what Marvel/DC decide about the trademarks....