Q:

Translating "If (request), that would be great."

In English, I often use the formula, "If (request), that would be great." 
For example:

If you could go to the supermarket after work, that would be great.
If it's possible for you to finish that project today, that would be great.
If you could let me know when you land at the airport, that would be great.

What would be some natural ways to express these sentences in Spanish?

A:

A good general pattern that you'd probably want to use, might be:

Sería bueno si pudieras Y. => It would be great, if you could Y.

...where Y is the requested action. (Note: In some cases, the Sería bueno could come
after the si clause, but both are equivalent.)
Examples:

Sería bueno si pudieras ir a la tienda despues del trabajo. => If you could go to the supermarket after work, that would be great.
Sería bueno si pudieras avisarme cuando llegues al areopuerto. => If you could let me know when you land at the airport, that would be great.

This pattern can be further generalized by replacing bueno with any other adjective that could describe the result (e.g. malo (bad), triste (sad), etc.) and by making the si clause any action in the imperfect subjunctive. (See: If-Then Clauses in Spanish.)
Examples:

Sería muy triste si ella no quisiera ir contigo. => It be pretty sad if she didn't want to go with you.

I hope this helps!

A:

I would translate the expression to 

Si (petición), sería grandioso.

or

Si (petición), sería fantástico.

