As a Dawoodi Bohra muslim, I believe that the true and rightful sucessor to Syedna Mohammed Burhanuddin (ra) is Syedna Mufaddal Saifuddin (tus).
Syedna Mufaddal Saifuddin (tus) was named heir at least three times, twice privately with multiple witnesses and once publicly in front of thousands of people at Raudat Tahera in Mumbai. There is ample documentation of these events, in form of signed papers, living witnessess, and video.
No Schism, just sadness: A Dawoodi Bohra responds (Patheos.com)