Definitely agree that their are differences in terms of how IM will be practiced in the future. Now the chances of matching into IM subspecialties vary depending on the residency program one matches into. So what may seem like a "powerball" fellowship as a DO student seems totally possible once that student matching into a well regarded residency. Here is a general listing of how the tiers work for residency:
top tier university program > mid tier university program > low tier university program > communo-university program >> community program.
The higher on the list you are, the better your chances of matching into a competitive fellowship. Coming from a DO school, one is an underdog in terms of getting a competitive residency. So if you look on the NRMP fellowship match report matching you will see that DO students (77.6% match rate) match at a lower percentage than US MD students (86.7% match rate). This difference again is attributed to DO student in general matching into a lesser tier residency, not simply because of DO discrimination (I have a feeling an attending or resident will counter this statement, hence why I put 'not simply').
http://www.nrmp.org/wp-content/uploads/2015/02/Results-and-Data-SMS-2015.pdf
Now for the list, this will be dependent on where you are in the residency chain and very little about whether you are a DO or not:
**(spot : applicant ratio)
Easy: Nephrology (1 : 0.7) , Infectious Diseases (1 : 0.8), Geriatric Medicine (1 : 0.5)
Normal: Endocrine (1 : 1.2), Rheumatology (1 : 1.2)
Hard: Cardiology (1:1.4), Heme/Onco (1:1.4), Pulmonology/Critical Care (1:1.5)
Legendary/Powerball: Gastroenterology (1 : 1.6) ( <-- kind of messing with
@AlteredScale since he likes Gastro, it is probably in the realm of hard)
DO have a near impossible time with matching into top tier IM, but have done so in mid tier university programs. So if you match into a mid-tier program then even gastro is doable.