Of course, it really comes down to what you believe about prayer, and the range of beliefs on prayer is very broad. One can easily find oneself praying all the time for everything, while getting nothing done. One can just as easily excuse all prayer. It is hard to know where to stop.
Also, it is not really my place to say someone "should not" pray for any particular thing. Prayer is a personal, private issue. It arises not only from what we are told but from what we feel. Prayer is not a repeating of things we are told, but is to be a conscious expression of our hearts. We will sometimes pray for things others find foolish, but so what? Prayer is not about the person for whom you are praying. Prayer is your communication with God.
This is important because we are sometimes accused of saying people "should not pray" for certain things, when our real focus is on what they "should pray for." If we emphasize praying for someone's spiritual health, we are accused of saying no one should pray for their physical health. On the contrary, we just want prayers to be serious and scriptural. There is nothing wrong with praying for someone's physical health, but it is clear that their spiritual health is much more important. You should pray for what matters in your understanding of the situation.
Anyway, let's begin (for this note) with John's promise that we know we are heard if we pray in accordance with God's will. And this is the confidence that we have toward him, that if we ask anything according to his will he hears us. And if we know that he hears us in whatever we ask, we know that we have the requests that we have asked of him." 1 John 5:14-15. So, we begin with needing to consider what God wants in your husband's life. Praying for what you know to be God's will is the best prayer of all.
God's desires are pretty well known to us. He has said that He wants us to be holy and sanctified in our sexual lives. 1 Thess. 4:2-8. You can certainly pray for your husband's sanctification in that regard. Of course, you also have to make sure that you are not contributing to any failure on his part through denying him sexual satisfaction in his marriage. I am afraid there are a lot of women who pray for their husband's sexual purity while they create the conditions for sin by their denial of their husbands. 1 Cor. 7:1-5. A wife who rejects her husband on Monday and prays for his sexual purity on Tuesday is acting foolishly, like a man leaping from a cliff and then praying that God will grant him a safe landing.
You can pray for the development of the fruit of the Spirit in his life. Galatians 5:22-23 gives you a list. Be aware, of course, that many of these characteristics may be developed by God through suffering and hard times. However, we know that God wants these things to develop in your husband and can pray for them with confidence.
You can look to the prayers of Paul for guidance. Colossians 1:8-10 is a good place to start. Ephesians 1:18-23 is another good source.
Pray for the faithfulness of his witness as he works out in the evil of the world. Pray for his maturity and patience and understanding of others. Pray that he will manifest his love for God and for his neighbor in all that he does.
And pray that he will always know and be certain of his wife's love. You cannot know how important this is in the life of any man working out in the evil of the world.