This is a good question that I think we will get a definite answer to next week in the gear stream. I want to say some of the "towers" in the Multiverse​ are static (offline) and will probably offer their own gear rewards, where as some are kind of like the living towers from MKX and do require an internet connection however I don't believe you have to have plus/live for those. Can anyone confirm?
Edit: also on the topic of guest characters, my friend and I had a very long argument about this lol. He is a casual player and said his favorite thing about MKX WAS the guest characters which disgusted me hahaha. So you can see why they're doing it, casuals drop money like crazy on them. Personally, I'd be fine without them. Oh, and he also said fatalities should be in Injustice. At this moment I chose random select in I1 and beat him to a pulp to pay for his transgressions.