“We should do this again sometime soon.”
You had lunch with a friend of yours who you haven't seen in a few months. You are finishing your meal and getting ready to leave. You want to be polite but you don't immediately want to plan another meeting with him, so this is your suggestion.
We should do this again sometime soon.
Suggesting something by saying "we should ___" is good when you want to make a general suggestion. In the example above, you don't want to specifically schedule another lunch with your friend. You just want to say that meeting again would be nice.
Some other examples of using "We should" to make a vague suggestion:
Hey, we should start a band!
We should start thinking about when we're going to take our vacation.
"Sometime soon" means "soon" but not at any specific time. You can use "sometime ___" with different time periods. For example:
Let's meet again sometime in the next week.
I saw her sometime this morning.
I'll get to work on it sometime after I get back from my trip.