Well, to phrase things in a respectful manner, it doesn't provide much for the game.
The Mursaat were largely faceless antagonists, with only two or three in the entire game every speaking any lines. They're main asset is their mysterious nature, right down to their title of Unseen Ones. Each Mursaat is an extremely potent enemy, with individual missions dedicated to assassinating each remaining one in the Krytan Civil War, along with an entire quest chain devoted to freeing a man from the curse of a single enemy (who, by the way, promised doom upon all humanity. Not a good selling point). Mursaat are powerful and mysterious, and that makes the reveal of their mystery all the more interesting as a potential plot point in the future.
Playable races aren't mysterious, since they have to allow a player perspective to guide one of their members. Likewise, they have to be approximately equal in power to the other races, since no one should have an advantage or be more special. But if you remove the might and mystery from the mursaat.........nothing remains. Nothing recognizable, at any rate. The description of using younger members of the ancient species who have now decided to ally with former antagonists for the greater good of Tyria is rather trite, since the same could be said for ANYONE. Demons, undead, krait, wardens, oni, shadow army, margonites....pick anyone we've ever fought, and the "now they're allied, and the younger ones are playable!" label could be applied to them. But it doesn't mean anything if it doesn't contribute something to the game.
The Unseen Ones should remain unseen until the ultimate truth is revealed regarding their conflict with the Seers, and that should lead to the grand conclusion of their place in the story. Playable races should be the more understandable, more relateable, more humanlike species, and dragging the Mursaat down to that level would only diminish the mystery and might which is so important to their character.
I agree that the most intriguing aspect of the mursaat is well...their intrigue lol, but in regards to their power we still don't actually know what the average level of power is, just as logan and other members of destiny's edge were capable of doing things no average human could (like nearly killing kralkatorix) the mursaat we encountered may be simialar as a sort of god among men, if you will.
I think that by having certain mursaat elders to guide a playable character through their early areas would be the perfect way to clarify the mursaat and their abilities, they may even adding in MORE mysterious elements, ie: more questions could pop up than those answered, giving them more depth and subsistence. In regards to Lazarus I think he could be useful as he provides a splinter element that are at odds with the majority and desire vengeance, that would be an amazing story I think.
On to your next point, I disagree that the same could be used to bring in any
new races In fact I would say that it could only be used for a few, namely free races that are capable of adding more to their numbers, as the mursaat are mysterious we know nothing of their origins, on the other hands things such as wardens, margonites, etc. are no longer being created, and for that reason are unlikely as they, like the dwarves, have a finite supply. Similarly creatures with only semi-sentience such as the krait, skritt, oni (they are likely insane), etc. can't really become playable as they lack the intelligence to carry out missions, complex plots, etc. Another group you have listed can best be defined as eternal servants, these creatures in organizations like shadow army, branded, etc. are incapable of breaking free from their masters and as such would never be playable as they are forever trapped.The only creatures you listed with any chance of being playable are really undead (in the form of liches most likely) and we can already play as a lich if you are a necromancer, as you can see although at first glance it would seem that almost anything could be playable in reality, due to the mursaat's unique situation they are really the only creatures that could logically become playable lore wise.
Regarding your final point I think it would be more impressive if the new races of Tyria were guided and aided by one of the last ancient races, it would be an amazing way to develop their race and give the current allies a fighting chance against the dragons and their minions.
Note: I didn't say that these would in any way be mursaat near the power of the elder ones, merely ones who would have an interest in the Tyrian struggle.