Toyota do have a pretty full line up but there's still several areas that they haven't enter, such as the big trucks and SUVs area. Tundra is nice for a first step but there's still bigger trucks and SUV that Toyota doesn't have an answer to.
Honda seems to me like they'll be staying in their area for quite awhile. They're great at what they do. Such as sedan, coupe, sporty car, van, and small SUV. But Honda did state that they're not planning to develope a V8 any time soon and segments that Honda doesn't have a car in require some sort of V8 with high TQ output. So they can continue to grow, but so can Toyota.
I agree Nissan is still in recovery mode. They were at the edge of bankruptcy only a few years ago. You can't turn 180 degree that quickly. In term of quality, as of right now, they're still behind Honda and Toyota. This is also due to the massive cost cutting to get as much profit as possible. They quality can only go us when they have more money to do so. Even with poor interior quality of the Altima, it's still selling like a hot cake. It might not out sale the Accord or the Camry because they can't produce that much anyways. Ever since it came out, month-to-month, the sales of the Altima has always been increasing. However, back to the main point, the fit and finish of Nissan right now is still behind Honda and Toyota.

Just my $.02