No one they could collectively add right now would give a 10-15% boost in probability of a playoff appearance.
Right now, the Big 12 has 10 of 66 power conference spots (we'll just assume the playoff spots will come from the power conferences). That's a 15.2% chance of getting any one playoff spot, although there are actually four. There is, then, an 84.8% chance that they don't get each of the four playoff spots (ignoring qualitative strength for now). Since there are four spots, that's equal to 1 minus (.848 * .848 * .848 * .848) or 1 minus .517. That means there's a 48.3% chance of the Big 12 getting a playoff spot currently, if we assume all four are coming from the power conferences and we don't adjust for actual strength of teams.
Add two more teams, we increase the pool to 12 of 68 spots. That becomes 17.6% and 82.4% chance of not getting one. That's 82.4% raised to the fourth power, which equals 1-46.1% or 53.9% chance.
So by adding two schools, roughly they've only increased their chances by 4.5%.
Honestly, that assumes any school they would add gives them an average chance of getting a playoff appearance. The only school right now from the non-power conferences they could add that seems to give a better than average shot is Houston, and who knows if that will continue. Tom Herman has the program in a great spot, but we'll see if they continue at that level very long.
I think they have to expand right now if only because they need the appearance of togetherness, ambition and strength. However, if we assume the ACC is off-limits right now, I don't know there is much out there that can solve their problem(s).