Thank you everyone for your suggestions.
It's interesting to see that lots of different types of names 'go' with Marissa. I think because Marissa is relatively uncommon and quite frilly, I'm conscious of ensuring that the next name is as special and not seen as more boring/common. (Speaking as a younger sister with the more common/boring name!)
I do like lots of the suggestions, her Daddy's name has already been mentioned, and some other names I like are used by friends/family which puts me off. I'm afraid I have a bad association with a Tamsin I knew when I was younger which has stuck with me!
I like Lucas/Luke but not sure it sounds right with my surname. Also like Hayden and Dylan but also doesn't work with surname because of the n (my surname ends in an n)! Claudia is really pretty but the meaning puts me off. Love Jasmine. I used to like Annabel but my DD's baby Annabel doll puts me off now!
I'm also thinking Leo, Rory or Zachary for a boy, but DH doesn't like Zachary. I think I like more popular boys name than girls names for some reason.
Any more suggestions are welcome, I have plenty of time to decide!