Texas would be my preference, the Dallas / Fort Worth area to be precise. Although locations such as San Antonio, Houston, or even Austin have things to offer as well.
Spring weather here is generally good -- though I wouldn't rate it as "Florida good" -- and should offer a warmer weather escape from those living in the north. Dallas and Fort Worth are away from the Gulf of Mexico though and don't have their climates moderated by maritime influences as much as Houston or perhaps even San Antonio do. What does that mean? It means that Dallas and Fort Worth are typically cooler in Spring than are Houston and San Antonio. There have been exceptions of course but this seems to be the general rule.
Texas is at least pretty centrally located and it's generally easy for those flying to get here without huge headaches. Dallas / Fort Worth and Houston are served by major airports that have direct flights from many markets. Costs of hotels may be a little better, although I haven't researched it, during this time as Texas doesn't seem to be a hot Spring Break destination with the exception of the Gulf Coast, the Padre Island area in particular and maybe Galveston (though that could be a bit of a reach). If any other Texas residents reading this have differing opinions please say so.
Venues to consider in Dallas:
Palladium Ballroom http://www.thepalladiumballroom.com/
Granada Theater http://tickets.granadatheater.com/
The Door http://www.thedoordallas.com/index2.html
Venues to consider in Fort Worth:
Ridglea Theater http://www.ridgleatheater.com/
Venues to consider in San Antonio:
(note that I've not been to either of these but they do host national acts)
Scout Bar http://scoutbarsa.com/contacts.phphttp://
The White Rabbit http://www.sawhiterabbit.com/main.htm
Venues to consider in Houston:
FBI Rock Club #1 http://www.terranova9.com/fbimusic.htm
The city of Austin has a very good music scene although I generally do not frequent it so I can't comment on what venues would be appropriate for the next installment of this fest. I'm sure there are some Austin experts that frequent this forum so hopefully they will respond.
It's also very likely that John (J-Man) or Tammy might have additional thoughts and suggestions about "the Texas scene" and if it might make sense to consider Texas as the location for Nightmare Metal Fest II.