For those who don't know yet, the UNCAF Nations Cup is a biennial football tournament for the member asssociations of UNCAF (Union Centroamericana de Fútbol).
There are 7 nations in UNCAF (in alphabetical order): Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua and Panama. With 7 teams, they always have a group of four teams and one of three.
In 2007, UNCAF considered inviting national teams from outside Central America to participate. They did start negociations with Mexico and Venezuela, but it did not happen.
If UNCAF still want to have 8 teams in the 2009 edition, let's see who should they invite.
I don't think it's a good idea to have guest teams - sure, they could raise the interest level of the competition, but they have nothing to play for as they won't be fighting to qualify for the 2009 Gold Cup as the seven UNCAF nations.
James Lagrange suggested inviting the Caribbean champion (winners of the 2008 Digicel Football Championships) as it could create some rivalry with the UNCAF nations. However, they would be already qualified when the Nations Cup kicks off in Panama on January 22nd, 2009.
How about inviting the 5th placed team from the 2008 Digicel Football Championships and give them another shot at qualification? It's really up to UNCAF - they could have a more interesting tournament, but at the same time they could risk losing a Gold Cup spot if the Caribbean nation makes it into the top 5.
Since there are two groups of four teams and the top two from each group qualify for the 2009 Gold Cup, there would be a play-off involving the third placed teams. Last time around, it would have been Guyana vs. Martinique. Now you could have Cuba vs. Martinique (looking at the Caribbean ELO Ratings).
Assuming Cuba wins the play-off, you could see these groups in Panama:
Group A: Panama, Guatemala, Cuba, Belize
Group B: Costa Rica, Honduras, El Salvador, Nicaragua