Final Fantasy VII: On the Way to a Smile - Episode: Denzel

Both anime that provide additional content to games in the Final Fantasy series. On the Way to a smile works as more of a side story focusing on a supporting character of VII Denzel whereas Kingsglaive acts as a direct prequel to the plot of XV it also doesn't focus on any of the main characters in XV and instead tells the story preceding theirs with plenty of tie ins and cameos being present in both the game and film. 

Both provide additional lore to mainline final fantasy worlds. On the Way to a smile focuses on a side character of 7 Denzel whereas Episode Ardyn gives extra back story to 15's main villain and the series namesake Ardyn Izunia. Both series will appeal to the exact same audience, FF fans who appreciate additional anime content. 

I believe these are both by the same studio, share similar animation styles & both provide added lore to popular mainline FF games. Denzel offers a post-game side story of sorts to 7 whereas Brotherhood is a prequel to 15. Fans of the franchise will enjoy their time with both.  

Both OVA specials that give additional animated content for mainline Final Fantasy games which provide the world with added Lore and give us a closer look at some of the series characters. On the Way to a Smile focuses mainly on a side character Denzel from the fan favourite Final Fantasy 7 (VII) whereas Final Fantasy (OVA) works more as a direct sequel to the events of the game Final Fantasy 5 (V). 

Both take place in the same world/canon. Both provide additional story and content for that world. Both feature some of the same characters.  

They both tell side stories that aren't necessarily important to the overall plot of the main series but work rather nicely.