Some more tidbits on early Imagawa Strategies (@ Segoku Jidai).

I can't get Takeda to accept an alliance no matter how many times I offer, it seems. I think that might have something to do with his wish to invade Suruga. However if you abandon Suruga he'll accept a ceasefire and alliance afterwards, which can be still long before his first heir comes of age (so you can assassinate him for a chance to get his lands).

Hojo seems a better target, although he'll often refuse an alliance early as well. I just kept trying until he did (I'm convinced it's partially determined at random), while producing ninja's and moving them to Shinano (Hojo's Daimyo will sit in Musashi early on). I think he accepted on the third or fourth try, iirc I had 7 ninja's sitting ready (produced 1 in Totomi every turn), then immediately assassinated him and gained all 6 of his lands. Pretty spectacular. Keep in mind you need garrisons in Shimosa and Kozuke because of loyalty early on.

I tried Shimazu too but it's harder. He always accepts an alliance, but the route your ninja's have to take to his daimyo (Bungo) is longer, so you can send less at him before his heir comes of age.

To expand upon my earlier post, both the Shimazu and Hojo heirs seem to come of age in Summer 1532.

Btw: In trying to test the Strategy of going for Shimazu, I ended up reloaded the game a couple of times, to the turn where a bunch of ninja's make their attempt (at 7%). I didn't get a successful assassination after what totaled to like 80 attempts. Seem like either the chance shown (7%) isn't accurate, or it's not purely random. It could be that some factor that is already decided goes into the calculations. I would love to know. I thought the investment wasn't all that bad, 5-6x 7% is a significant chance. I should replay that opening and see if the same phenomenon happens again.

Meanwhile, the Hojo Daimyo (also 7%) fell to the first ninja I sent at him.