Oh boy. First time scrolling through concepts in awhile and I see this. My first criticism is how you give 1 sentence explanations for the names of the more obvious ones (like Boston and Toronto) yet for say Sacramento and Charlotte no explanation. Though I like to figure out reasons for names though.
Sacramento Kraken: Oh boy. Sacramento, one of my favorite cities to see concepts for and Kraken, one of my favorite names. Together. Why? THeyre not near the Bay or any ocean. There is the Sacramento River, but then something like River Monsters could fit. But hey, this is football and we see weird names all the time. As for the concept its pretty good, though gives me Sparky Chewbacca vibes (probably why you choose Sacramento now that I think about it) curious to see their uniforms as the colors are an interesting pick.
Toronto Lakers: Good logo, good team name. Not really much to say. Color scheme is good.
Boston Mariners: Probably one of the more solid ones here. The purple and light blue have always been a favorite of mine. Wonder how you'll implement the logo on the helmet
Houston Voyagers: Literally nothing to say here, except, please let the helmet logo be the space ship.
London Royals: I kinda was hoping for a more unique name, however if the NFL moved there I see no reason for them to not do a royal/monarchy theme.
Winnipeg Blizzard: I'm assuming the token white-at-home team? Almost always a favorite. Though unsure how Winnipeg got a team if only 6 Canadian cities got a team.
Seattle Shockwave: Uh, not sure why but the name bothers me. But MJWalker's point also stands.
St Louis Legion: I have an idea for their helmet design but I won't mention it because its impractical.
Charlotte Flyers: ... Carolina Flyers fits better than Charlotte as Washington DC is closer to Kitty Hawk then Charlotte. Though if the logo was designed based off of cars "flying through the track" it'd make more sense for it being Charlotte. Really the only one I have major problems with. Also 2 teams with primary purple seems odd, even if the shades and secondaries are different.
LA Tides: Most underwhelming, it'd be fine if it was anything but LA. LA is either an already established brand or something extravagant not 2 tone blue.
Baltimore Warriors: The B in the Baltimore flag shield looks off with the way the pattern is, slight adjustment would help. Like the logos.
Buffalo Polar Bears: Another 2 tone/mostly blue team? I just want to see the jerseys for them. I always liked the name Polar Bears or Polars but never really thought about it besides hockey in some capacity