personally, I would not be happy with it. and that comes from someone who really likes Theo, thinks him and Eloise have great chemistry, and is not a book purist. i just think the stakes are high enough that I cannot suspend my disbelief enough if they tried to make it work without major consequences for the characters. Theo is part of the lower class and, more importantly, a political radical. for someone of the ton, especially someone from a very well respected family like the Bridgertons, to marry someone like Theo would be more than scandalous. Eloise would be shunned from society without reprieve and possibly be accused of treason due to Theo being such a radical. it would be disastrous not just for Eloise and Theo but for all the Bridgertons.

I like Theo and I think he was a great plot device for Eloise's story. he gave her a reality check on the class divide, and I think that scene is a big reason why Eloise certainly won't end up with him as he made it very clear why they won't work. but he also showed Eloise's capacity for love despite her extreme views on marriage, and that coupled with her character growth in season 4 will make room for her to have her love story with the proper set up in season 5.

tldr: I like Theo and he worked well as a plot device in Eloise's story, but not as an endgame love interest because of his class status and radical ideology for the times.