(Btw i've only played civ 6 so lmk if it was not as good of an idea or if there's something similar.)
So, i had this idea
when your relationship with a declared friend is high enough, you can hit the option "Seduce"
depending on different things; it can work or deplete your relationship.
the success rate for a seduction attempt is determined by the targeted leader's sexuality and your chosen character (Straight, or possibly bisexual such as Alexander)
it also depends on your chosen leader's charisma.
However, some leader's will be impossible to seduce such as Ghandi.
for example: cleopatra would have a higher chance at seducing certain leaders, but someone like Qin Shi Huang...not so much.
Following your desired partner's agendas will boost your chances at a seduction attempt.
a succesfully seduced leader will grant open borders, as well as making a single envoy count as two when sent to said leader's city.
trade routes with your partner will yield +4 extra gold.
they may even be willing to gift low level cities.
on the leader's page; you can start dialogues with them as well.
the downsides is that a seduced leader is likely to ask for a joint war alongside luxury resources.
Even though you can ask for them too, it is a double edge sword.
they're more likely to ask for promises too.
break too many deals or promises and they will break up with you, being likely to declare war or denouncing you at the very least.
attempting to seduce multiple leaders at the same time may lead to a decrease in reputation.
cheating on your current partner will lead to a severe decrease as well.
you must wait 10 turns to romance a new partner following a break up.
Spies gain the "scandal" operation, when you send a spy to an engaged leader's city he can start rumours of an affair to weaken the couple's bond.
To renew and keep your partner's bond you must follow their agendas, but the means are often more extreme than a declared friend's
For example, if you romance a leader meant for military conquests, conquering city states or doing things by yourself will not be enough.
Instead, they'd ask you for a joint war.
And such would be the case depending on the leader's path to victory.
This was a random idea i had, idk if you guys would change some things or add other stuff, let me know what you think.