First, I know this will never happen, because I doubt GameFreak reads this. But anyway, I thought of what I think is a pretty good way to keep the number of legendaries introduced in Gen 5 to a minimum.
First, instead of creating an all new legendary trio, I say B/W should use a duo from an old game. In this case, I think it should be Darkrai and Cresselia. Add a third legendary Pokemon that relates to dreams and the moon, and make it a Fighting Type. That way, it'd create a Psychic/Fighting/Dark triangle, and also give us our first Fighting type legendary. Make this trio the legendary trio for B/W, instead of creating three new Pokemon.
Also, I think Heatran should be stripped of its legendary status. B/W should explain that Heatran is close to extinct in Sinnoh, and it's why people in Sinnoh consider it a legendary Pokemon. In B/W's region, Heatran should be rare, but not legendary rare. Fifth gen should introduce a pre-evo or two to Heatran and turn it into a Pseudo-legendary instead of a full out Legendary Pokemon. Also, somehow explain how Heatran can suddenly breed. I donno how they'd do that.
That's just a few ideas I had. Stripping Heatran of its legendary status, and then adding a third to the Cresslia/Darkrai duo to make it the trio of B/W wouldn't raise the number of legendaries at all. Of course, they'd add some more, like the tiny legendary, the event legendary, etc. etc., but at least it'd prevent them from making three more legendaries than they need.
I'm sure someone will come in here and tell me how broken and bad these ideas are, but oh well. Thoughts, opinions?