Interesting idea but think of game balancing. Making bonuses tier based on several conditions is more complex than what dev has decided is necessary. If that kind of bonus came to pass, players may start pushing for edits to bonuses for generals who have back story interaction with each other…which would lead to consideration of game balancing and rescripting of parameters and dependencies. Not to be snide, but have you considered why the Demi-god Generals don't have equipment bonuses? Not a knock on you, but just my two cents on the topic as a hobby game dev ^^
If you think about it, the idea for the General Bonus for both Tristram (which my actual name is derived from btw, lawl) and Isolde is linked to their little legend. Wouldn't it make sense for two lovers to empower each other? Plus it's a cute little gimmick to try and get both of them (who knows how many favor points you'll spend to achieve that buying chests). That said, it's one of the more interesting bonuses I've seen in the game.
With the influx of new FP generals, the developers are eventually going to need to think of a way to keep money (as in we pay for FP) flowing in without destroying relative balance of the game. It wouldn't be surprising if they altered "magic" to inflict ailments on opponents, which would pull away from looking to have high stats in a couple areas to knocking an opponent down to your level. People who play RPG's kind of know that buffs and debuffs have their uses. However, the prospect of a bonus for a general such as "Increases damage against Dragons" would prove relatively valuable compared to a raw stat boost. And in doing so, it gives a comparative edge to players without destroying balance overall (which, I presume would reside mainly in the PvP activities the game has). Of course, that kind of bonus would have to be a relatively small percentage (probably starting at 2% then maxing at 5%, which seems small but if your base damage is 15k, then 15k + (.05*15k) is a relatively noticeable difference). And naturally, there would eventually be generals giving bonuses for other kinds of Epic Battles/Monster "types".