Who's on the bubble is a function of how much roster space they can clear out.
I spent some time this morning looking at the roster, and the list of Rule 5-eligible prospects. Depending on how many relief pitchers they're willing to just outright, they can protect everybody they probably need to, but only if they're willing to outright a large portion of their current reliever corps. It looks like the last roster decisions will involve pitching prospects: Elniery Garcia, Jesen Therrien, Ricardo Pinto, Alberto Tirado.
How did I get there? Looking at the current roster, and considering that they'll need to build a 25-man roster in March/April (and assuming no additions due to trade or FA signing - obviously, this subject to change!).
Twelve starting pitchers: Asher, Eflin, Eickhoff, Harrison, Hellickson, Morgan, Nola, Thompson, Velasquez, plus add Appel, Lively, Pivetta. Five of these guys are in the April rotation, six are in AAA (if all are healthy); this will be very interesting in the spring. Matt Harrison is DL'ed. Also, this assumes Hellickson accepts a QO; if he doesn't, that would open a roster spot. Outrighted: Buchanan (he's out of options, and about tenth in line among the starting candidates; might as well outright him now as in the spring).
Eight relief pitchers: Seven on the Active roster in April (five RH - Garcia, Gomez, Gonzalez, Neris, Ramos; two LH - Rodriguez, Araujo); one optioned to AAA - Cordero. Outrighted: Herrmann, Hinojosa, Klein, Mariot, Murray, Schuster.
Three catchers: Rupp, Alfaro, Knapp - one optioned to AAA (or two if they pick up a veteran at some point - but that consumes another roster spot).
Five infielders (Joseph, Hernandez, Galvis, Franco, plus a backup - Burriss/somebody else)
Nine outfielders: Five active (Altherr, Asche, Herrera, Paredes, Ruf - or likely somebody else replacing Paredes/Ruf, neither of whom has options remaining); four optioned to AAA (Cozens, Goeddel, Quinn, Williams).
That's 37.
So you've got three slots, and four vulnerable pitchers (Garcia, Pinto, Therrien in AA, Tirado in A-ball). They could lose a slot if they decide to sign a veteran catcher to pair with Rupp. They could gain a slot if Hellickson leaves. They could gain a slot if they're planning to promote an outfielder (Quinn?) directly to the 25-man roster. They could gain (buy) a slot by releasing Matt Harrison.
So they can make this work.
They would be exposing a bunch of AAA and AA players to the draft:
Featherston, Moore, Mora, Perkins, Stassi, Sweeney, Valentin from AAA;
Canelo, Curletta, Hollands, Leiter, Lino, Milner, Martinez, Nunez, Pullin, Walding, Windle from AA.
(And yes, Carlos Tocci, among others, from A+.)
They could be more aggressive at purging mediocrities - they could outright Asche, Burriss, Paredes, or Ruf, for instance. But this doesn't really help them; it could free up a roster spot in late November for a prospect...but they'd only have to add a player in the same role in December-March, and when they did, they'd have to waive one of the prospects they protected in November. It's easier to keep a prospect claimed off outright waivers than a Rule 5 pickup - the waived player can be optioned. Better to take the risk in the Rule 5 draft.