Don't I know it
I'm pretty sure they got a budget bump for the second season (which is good). And I guess they always wanted to have more regulars since it's all about "Team Phoenix".
Thing is - like you said - that MacGyver doesn't get enough screen time as it is.
They wanted Team Phoenix (or maybe had the order from CBS to do a team show). Yet they still named it "MacGyver", promoted it as a 2-person-buddy-show à la Lethal Weapon (with all the Mac-and-Jack-Hashtags) at first, then switched to MacGyver-only promo (Mac is back). Additionally, they only have MacGyver (or at least his hands) in the opening title sequence. It's still marked as a one-man-show which is confusing for fans of the original show or people watching the reboot without knowing anything about it.
Alas, so be it. More season regulars.
No matter if she's coming in as a love interest or not (which I think she will be eventually if not right at the beginning), it will change the team dynamics.
I'm not sure if that's a good thing or not. Bozer already changed the team dynamics. He was written as "the best friend" for the pilot and clearly intended to stay the clueless "alibi black guy" that bookends the episodes. Then they realized they wanted him as a full team member. All the established storylines in the first few episodes "vanished" with no explanation (e.g. Bozer posing as Riley's girlfriend for her parole officer, his movie career). Some were tied up later on (e.g. Riley getting the last few digits of his phone number) after they had neglected it.
In the first few episodes, Riley was always with Mac and Jack on missions (despite her not being a fully trained agent yet). When Bozer joined, he suddenly was on missions, too - which makes no sense (he's not trained, either. also - why do you need a guy to make masks on every mission?). So, they paired him up with Riley and they do other work in the meantime. It makes sense since Riley can do most of the computer stuff from the base anyway.
Since this new character is a "real" agent, I guess she will go on the missions with Mac and Jack while Riley and Bozer team up again for side-missions. We can bet on drama stuff happening if both are dating other people (as Lenkov implied in a recent interview).
But who knows. Maybe she's MacGyver's half-sister or something. I'm still waiting on a plot twist concerning MacGyver's father and I'm already dreading it. Either he was a secret agent as well or he had another secret life...
