I have to admit I initially had no interest in Suits and had no idea who Meghan Markle was before she started dating Prince Harry, and the only reason I originally decided to watch this show was because I was curious about it since Meghan was in it and I initially watched this for her. I binge-watched all 7 seasons recently and I got hooked on it almost instantly, Meghan may have been the only reason I decided to watch the show in the first place, but it was Gabriel Macht and Sarah Rafferty that kept me hooked on the show, I love Harvey and Donna, they are both my favourite characters on the show
I also like Louis a lot, he's such a goofball and interesting character, even though he's very naive and his childishness can be a little annoying sometimes, but overall he's such a funny character and I enjoy watching him most of the time. I used to like Mike in the beginning too, but after a while his character started to annoy me a bit. And Meghan's character Rachel didn't really do much for me, she really wasn't a particularly interesting character to me, so I didn't really care for her as much as the other main characters. So I'm not particularly sad or disappointed about Mike and Rachel leaving the show, I think the show will definitely be different without them, but I probably won't miss them as much as other fans would.
As for Jessica, I liked her too but honestly, I still don't like her enough to watch her own spinoff, her own show focusing on just Jessica's character doesn't sound interesting to me, and I thought the season 7 finale which served as the backdoor pilot for her spinoff was rather boring, I wasn't interested in her story. The only storyline in the finale that I was invested in was Mike & Rachel's wedding, and there was not enough focus on their characters. Considering this was both Mike & Rachel's last episode in the show, you would think they would have more focus in the finale to give the couple a proper send off, but no, they were sort of pushed aside as the secondary characters while most of the focus in the finale was on Jessica and her story, which I had no interest in.
As for Katherine Heigl joining the show, I've heard a lot of very negative rumors about her saying how difficult of a person she is to work with, but I'm not very familiar with her and I don't really care about her, so I'm not as bothered about her joining the show as a lot of other people are.