God willing, ...
 

I wasn't sure where this thread belongs, whether in Idioms & Sayings or Vocabulary, and sorry if it's misplaced here.
How do people from the various countries represented in Tomísimo say "Lord willing" either at the beginning or the end of a statement wherein they commit themselves to do something? I've heard the following, at least from Mexicans and Costa Ricans:

Dios primero
Primero Dios
Dios quiera
Si Dios quiere
Si Dios lo permite
Dios mediante

Are there others as well? Which ones are the most widely-used?

The one I hear is si Diós quiere. The Spanish I hear is Caribbean.

"Si Dios quiere" and "Dios mediante" are the most used in Argentina. There's a subtle difference: the first one is an independent clause and it may mean something between "if the natural course of things is what has to be" or "if the course of things is a bit better than what the natural one has to be"; the second one is an adverbial clause meaning that all random variables have to 'align' in a positive fashion, or at least in a way they won't interfere with the wished outcome.

"Dios quiera" is also widely used, but it's similar to "¨¡Ojalá!" (and it etymologically means almost the same): it's a rather propitiatory expression of wishes or hope, and something that "favours" the expected outcome.

I agree with Alec that "Dios quiera" is rather a synonym of "Ojalá", which expresses a wish that something will turn out as the speaker expects, while the others are a way to "exorcise" something that could go wrong in their plans.
Except for "Dios primero", which I have never heard, but seems to me a variety of "Dios mediante", all of them are mostly equally used in Mexico, and it will depend on the context and the person.

·Nos vemos mañana, primero Dios.
·Nos vemos mañana, si Dios quiere.
·Nos vemos mañana, si Dios lo permite.
·Nos vemos mañana, Dios mediante.
-> We have a certainty that we will meet tomorrow, unless something beyond our control will impede it.

·Dios quiera que nos veamos mañana.
·Ojalá que nos veamos mañana.
-> There's a big probability that we won't be able to meet tomorrow, but we wish we could.


·Primero Dios, este mes termino de pagar la hipoteca.
·Si Dios quiere, este mes termino de pagar la hipoteca.
·Si Dios lo permite, este mes termino de pagar la hipoteca.
·Dios mediante, este mes termino de pagar la hipoteca.
-> If nothing wrong happens with my money, by the end of the month I will make the last mortgage payment.

·Dios quiera que este mes termine de pagar la hipoteca.
·Ojalá este mes termine de pagar la hipoteca.
-> Something is going wrong with my money and I might not be able to make the last mortgage payment, but I wish I can.

¡Ojalá! is a Spanish loan word from Arabic. It literally means ¡By Allah!

More exactly, it is a corruption of wa sha' Allah = may god want! :)