Since the game is based around the Deathwing, I am fine with it just being Dark Angels for the time being. That being said, it would be cool to have additional campaigns down the line with more chapters and enemies available. Fighting some ancient daemon as the Grey Knights, cleansing orks as the Salamanders, or even fighting the forces of chaos and traitor marines with the Reclaimers would be awesome additions.
The reason I chose these 3 chapters specifically is for a number of reasons. The Grey Knights would have powerful psyker abilities, every class have a more interesting melee weapon like a Nemesis Halberd or Nemesis Falchions, has some pretty badass Terminator armor, and has unique ranged weapons as well like the Psilencer and Psycannon. Inquisitors could even be included with them too. Salamanders are unique since they all have combi-bolters, can extend the bones in their arms as swords, and they look very different with their dragonlike features. The Reclaimers would be interesting purely because they are significantly closer to the Adeptus Mechanicus than any other chapter, and because of this they have a unique relationship with them that gives them all sorts of advanced technologies.
For the time being, I would be fine with being able to customize your terminator with some form of heraldry or something that can personalize that armor. Terminators are veterans that earn the right to wear such venerable armor, and many of them retain it for decades or even centuries. With that level of commitment and ability, it would only make sense that they customize their armor with trophies, paint schemes, and so forth.
I am absolutely against the Ultramarines even being a thing, even the Novamarines are superior and are vastly more interesting.