I agree we don't know what will become available, unlike last year when we knew Harper and Machado were on the market. But as you say,Realmuto was a nice surprise acquisition. (Segura I think of as not so much an improvement but a necessary shuffling to get Hoskins out of LF. He brought more certainty than Crawford, and essentially we replaced Santana's bat with McCutchen.)
I'm hoping that there is a big bat to be had. Maybe Donaldson. I can't imagine the Phillies would pay Rendon's price. Maybe there's another surprise out there. Of course it would be better to improve the pitching, but it might be possible to do both by spending money.
I don't see Herrera, Williams, or Franco contributing in 2020 though one of them might still be on the bench.
As to your question about a huge deal for Hoskins, it depends on whether any offense is coming back and at what defensive position. If no offense is coming back, I would consider Bruce replacing Hopkins in the lineup to hurt the offense--even more so with Franco--(of course if the deal is "huge" and no offense is coming in return, that implies a big pitching upgrade, and so maybe an offensive decline could be accommodated). A question would be where Kingery plays. If Cesar is still around, that leaves 3B and CF--or maybe he could play some 1B. Or maybe one of the other OFs could play 1B, if we get another outfielder somewhere else.