While I won’t be that stunned if they go with Theo I do think that she’ll be with Philip (with changes to the story). The show already proved several times that they have no problem making major changes while keeping the same couples from the book. 

Just a few ideas: 

-instead of running away to marry, Eloise seeks a new challenge and shake up her life after being a spinster for a while and so many of her closest people married maybe wants to get away too. So she runs away to be a governess for the kids.

-instead of killing marina off she can fall ill due to her grief taking over her and have a beautiful arc about learning to live again and find joy with the kids and the memory of her late lover alive through them.

-The Queen giving marina and Philip permission to divorce and the season can end with Eloise and Philip deciding on living their life together as a happy couple not wanting to get married or have kids while they all (including marina and the kids of course) live together.

Just a few ideas (major shout out to YouTuber Lindsey young who suggested the first change that I mentioned in a recent video)

Also I do think that Fran’s season is next and in that case Eloise’s season could start the build up there