Daario Naharis
Introduced as a lieutenant of the sellsword company the Second Sons, Daario Naharis eventually became a trusted advisor to Daenerys Targaryen when he killed his superiors and helped her liberate the city of Yunkai. Eventually, they became lovers, and he helped her drive the Sons of the Harpy out of Meereen. Daenerys planned to sail to Westeros after winning the Second Siege of Meereen; however, she decides to leave Daario in the Bay of Dragons to maintain peace.
Including Daario in the final couple of seasons would’ve been interesting, as he could’ve come in with additional soldiers or mercenaries to help Dany fight the White Walkers or for the Iron Throne. Dany could’ve been desperate after losing a chunk of her Dothraki forces following the Battle of Winterfell and have no choice but to call in reinforcements from across the Narrow Sea. Daario’s involvement could’ve also given a love triangle between him, Dany and Jon.