My tips on obtaining more legendary heroes...

[Shampoo]

I beat the game already in Records mode and am in the process of beating the game on Romance mode too and this is what I have learned...

1) Trade with the faction that has the legendary heroes you want early in the game, it doesn't have to be deals that give them a advantage, just fair deals is fine. Just trading with them increases the faction favor for you. This will be important later on.

2) Vassal! Vassal! Vassal! The moment you reach the rank that you can obtain vassals, vassal the faction that has the legendary heroes you want! It would be best if you can reach that rank before everyone else, or else you may run into someone else turning the desired faction into their vassal... Like for instance I was playing as Cao Cao, and I immediately turned Liu Bei into my vassal when I reached that rank. My previous trade agreements with Liu Bei and my immense power helped of course.

3) Keep your vassals and the other factions low rank and POOR! By poor, I mean poor in gold. Dominate them through food trading. Make sure your vassals and other factions hold low ranks such as Second Marquis. Legendary characters tend to be hard to satisfy, if the faction is consistently poor in gold, they can't pay them off, and if the faction is low rank, they can't give them a title to make them happy. If the faction is always poor in gold, they can't field these legendary generals in battle and of course they suffer from the negative effects of "Lack of Purpose". This will encourage these legendary heroes to seek new masters, such as yourself.

4) When dealing with vassals it is encouraged to make sure you only make those who would not betray you into your vassals. You can check their faction personality for more details. Everyone is more likely to be your vassals if you trade with them often and accept their peace treaties when you have no intention of attacking them. Also make sure to guarantee that you won't annex them... (For now.)

5) Keep your vassals happy by trading with them often, trade food for gold! If they can't pay up in Regular Payments, TAKE ALL THEIR MONEY with Request Payment! Remember, a poor man is a honest man. This will increase your faction's favor with them and make things easier later on. Also whenever you are expanding, declare war on a enemy faction, and then have all your vassals declare war on that faction. So whenever you execute a enemy general, all of your vassals would get extra relationship points for "committing war atrocities against our enemies".

6) It's okay to be Untrustworthy or even Treacherous when you are at the stage of the game where you have tons of vassals and they are reasonably satisfied with your faction. For me, I don't mind paying a premium to get someone to be my vassal when I was able to, since I know I will be getting all that gold back later when I would be busy keeping them nice and poor. You need to spend money to make money!

7) My favorite step... ANNEX!! This should be done in the late game when the country is divided into distinct and powerful factions where there is little chance to make any more vassals. It is time to ANNEX! Absorb the vassal that has the legendary heroes you want into your faction! Don't worry about being Treacherous, as long as you had done the previous steps in making your vassals happy through food trading and committing war atrocities as well as keeping them poor in gold, you should still have way more green points in satisfaction when you are finished annexing. And the legendary heroes you gain will have 100 satisfaction because you were nice to them before... I annexed Liu Bei when I was half way through the Duke rank, because I feared when the Three Kingdoms period emerges, Liu Bei could become one of my main contenders and I don't wanna fight Guan Yu in battle, and I gained Liu Bei and Guan Yu. Don't know what happened to Zhang Fei, he is probably dead or defected long ago. Liu Bei and Guan Yu both have 100 satisfaction. Sure, Liu Bei has a -50 satisfaction cause he was angry about being absorbed but I have such a surplus in satisfaction for him, he still has 100 when he joined.