Integrating external tuners, especially from third parties like the HDHomeRun, seems extremely unlikely. I wonder how many people use the combined OTA + Cable at the same time in a Premiere, TiVo HD, or 4 tuner Roamio. There are tradeoffs, and cost is a huge factor. 4 OTA + 4 cable tuners probably cost about the same as 6 cable tuners. To keep from having to overhaul the scheduling code, they'd need to sell you as many OTA tuners as you have cable tuners (having an unbalanced number of tuners would complicate conflict resolution, etc). As constrained as TiVo development seems to be, I don't see this ever happening.
Current: Roamio Plus + Premiere
Former: S1, S2 TiVos, UltimateTV, SD DirecTiVo (x3), SA 8300HD w/Passport Echo, DirecTV HR-24 (x2) , DISH 722k, DirecTV Genie + 2 mini