Everyone is waiting for an engagement but what would happen if there never is one. Meghan has been there done that so might not be wanting to go there again. Would they be able to continue with their current arrangement? Meghan keeping her acting career etc.
They likely could, but I don't think it would be a long term solution.
For starters, if Meghan were to continue acting she'd likely have to be based out of the US at least part of the time (or Canada so long as she's still on Suits). While she's doing that any security she and Harry want her to have has to be paid for by the two of them. She's also going to be limited in the roles she gets - she'll only be getting roles from producers who are okay with her not doing any promotional interviews - and what ones she can take - she's not likely to be doing any nudity, sex scenes, etc. And that's in addition to the limitation that her age and race already put on her (she's spoken about how being biracial has made getting roles harder, and actresses who are much bigger than her have commented on how hard it is to get cast as you enter your late 30s/early 40s).
They could very easily have children - they just wouldn't be in the succession or have any titles. Harry wouldn't be the first British Prince to have illegitimate children or live long term with a woman who isn't his wife, nor likely the last. But he would have to be okay with his children having a diminished status in comparison to what he and others of his family have.
There is a huge debate about whether or not Meghan would be allowed to do things like Christmas at Sandringham if she and Harry never married - I think likely not during the Queen's lifetime, at least not if any children aren't born during her lifetime. Meghan also would never accompany Harry on any official engagements. I mean, sure she might be with him if he's in the audience at the Olympics or Wimbledon or something, but she would never be in a carriage with him at Ascot, or standing on the balcony at the Trooping, or with him at any one of the little day to day engagements that he might do. She definitely wouldn't accompany him on any official foreign tours either.
The sense that I've gotten from Harry is that what he wants is something like what William has - a wife and family with whom he can share his life and his role with. I don't think he wants to have just a partner in the background, staying at the palace while he does something for Heads Together or Invictus or a garden party or whatever - he wants someone who is going to be doing that with him, much like how Kate is doing that with William.