In a business sense, Roman for the Bucks would likely work really well for both companies. The shock value of Roman in AEW paired with him getting to be his own person there without the baggage of the past few years that somehow built to the point that him recovering from Leukemia can't even make him a straight babyface... that could be huge for him, and definitely draw eyes there on their main title scene, which I feel like they should be focusing on to the detriment of everything else, just to establish it before branching to women's/tag/midcard...
But the Bucks going to the E would basically force them to focus on the wealth of good tag teams they have that don't get their due because of Vince's hatred of tag team wrestling... There's no reason with the people they have there, the tag titles shouldn't be able to main event regular TV comfortably so you can have the Brock/part-timer reigns where they're rarely there and not have it gut your product. Bucks-Usos, Bucks-New Day, Bucks-Revival, hell... Bucks-Heavy Machinery would be a string of things they could build up and deliver and let the fast-paced, multi-man matches (Which, I would argue WWE's agents are far better at than singles matches these days) to shine in the spotlight and have your top title literally never see TV because it shouldn't, really. Make it special, and in the interim, bolster the other divisions.
It'd obviously never happen, but I believe this trade could literally alter both companies in one move. Would also force Omega to be on his own for the first time in years without a clique, and Roman's move could effectively tie the ubiquitous Anoa'i family to AEW as well, creating a market for all the spawn therein and making it so the better workers - the cream, if you will - can rise to the top. (RIP Macho)