WWE currently has several storylines planned and that includes who Roman Reigns opponent at TLC could be. The issue is that there are not many Superstars who could face him.
Daniel Bryan is currently the top face on Friday Night Smackdown and reports have suggested that he is likely being considered for a title match against Reigns at TLC. It is still an idea only and nothing concrete as of yet.
On Wrestling Observer Radio, Dave Meltzer mentioned that WWE is also considering postponing the championship match between Bryan and Roman Reigns to the Royal Rumble pay-per-view in January.
The match taking place on one of the major four pay-per-views of the year would certainly make it more special. Currently, no plans have been finalized as there is still a lot of time until the December 20th event.