Seems pretty sensible, right? Two more major series are brought up to three characters, and Mother receives a new face to accommodate its most recent release. Truthfully I wanted to try something different with Mother 3 representation, but I ran out of space. I also considered throwing in Kumatora instead of Lucas, but it made more sense to just let the PSI boys come together.
Dark Samus is pretty hot at this point, with Prime 3 releasing near Brawl's release and Prime 2 already having been out a sufficient amount of time. Adding them to Brawl assures Dark Samus is made largely unique despite working of Samus' basic moveset, compared to if they were to be added as a last minute clone or echo fighter in a future title. Meanwhile, Lyn is a popular face within her series and the first FE game that reached western shores. While Lyn has her own distinct fighting style, and this may prevent her from using her signature bow, I think it's reasonable to use Marth as a base similar to Wolf where specials can take inspiration from a common base and deviate almost entirely beyond that. Better now than never.
I like the way this roster is coming along, although I should probably acknowledge that Pokemon is a little flimsy right now. I'm planning for next newcomer round to be a big one for that series, and then I think the roster will have rounded itself out nicely.