To those that participated, nice lists. Glad to see many people feel as I do about Marth. I would like to talk about my other choices, as Mirron did.
I almost posted that with Celica at #2, but I went with Robin instead, with two reasons for that choice. The first being I dread they would use the redesigned loli doujinshi Celica, which is completely unacceptable. The other being that I feel Robin would overall standout the most from the cast, with dart motif and attire.
Tiki would be a cute addition, and I think she could offer a new breed of flying projectile user. Plus I would love to see divine magic/white flames. She would be absolutely nothing else.
Elincia, biased because she is wonderful. That said, she would be the prime candidate for integrating the qualities of a pegasus knight into a fighting character. Both because she is lordish, and because she can mix it up a bit.
Ephraim more than anything because he is without a doubt the best spear user I can think of. Other lords can use them, too, but I like if they stick with their marketed qualities.
I really, really want a Pegasus Knight Lancer/Dracoknight Axe-bearing Lord. I mean, c'mon. It would be so cool.
It becomes very difficult to design chapters if lords can fly. Originally, the lord being weak and immobile was a core gameplay element. You had to balance protecting the lord versus letting them fight so they can stand up for themselves, and guide them to the throne or fortress to seize it. Strictly speaking of starter lords (IE not Ephraim), with the exception of Celica and Sigurd, it was that way up until Ike, where they became less Myrmidon and more Mercenary. Well I guess Alm was a little Mercenaryish.
If they were to have a flying lord, they would need to drastically change how seize chapters are done. Just having the boss on the throne is not enough; there are already a number of strategies for FE8-10 and 13 that just have the player ignore most of the content with rescue, because they can survive the nearby enemies and kill the boss in quick time. While it would be new and interesting, if we wanted a flying lord we would really need a different kind of game.