I see people are still mischaracterizing why the Wars series no longer sells in Japan, so I will
provide a full refresher course on the subject.
That being said, there are ways to make the series more compelling beyond the character interactions.
For instance, the aesthetic of Days of Ruin (y'know, the game that cratered the franchise) went too far in the opposite direction from the other games in the series, but the series came across as overly lighthearted for a war game in the GBA iterations. I think it could stand to take some aspects of the lighthearted aesthetic and mix it together with a 1940s war-propaganda aesthetic, to add some weight to it without going so brutally in the other direction.
And playing the games on VC has led me to believe that between the some of the OP CO powers and the map layouts, the encounters seem impossible to beat except for one single method, which not even Fire Emblem suffered from. That can get very frustrating to some players.
And the gameplay could stand to be sped up a tad. There is a lot of time spent waiting.
But even if you want to make character interactions a thing, you could do something like alliances with nations, made into a game mechanic by certain nations having better versions of units that they would share with you based on your relationship with them and your performance against them in batter (eg. Yellow Comet could have better aerial attacks, Green Earth more resilient infantry, etc). This would also be more in line with what the Wars series is, a conflict of nations. Not everything is better with waifus.