As the title said, this is a theory about how you can revive a dead shardblade (a spren that isn't bonded to it's owner). This theory isn't yet complete, and feedback is appreciated.
Step 1. First you need to discover what type of spren the shardblade is. This can probably be discovered by watching the mist you see when the blade materializes. For example, Adolin saw mist in the form of vines when his blade materialized, which probably means his blade was a edgedancer spren.
(Words of Randiance chapter 50)
Step 2. The person attempting to revive the blade must have a cracked soul. This is important because when the blade is revived you will have bonded it the same way as you would with a normal Nahal bond. (Maybe using Hemalurgy or Soulforgery.)
Step 3. Change the way the spren is bonded, this is because a blade presumably is bonded the same way as a Seon, enabling it to bond to people without a cracked soul. Honestly, I'm not sure how this would work, but I think you would first need to change the place of the bond in the spiritweb so it is placed at the crack (not sure if this is necessary, This is some wierd stuff in the Spiritual realm). Secondly, you need to make the bond stronger. (Again, no idea how this could be done exactly, and if it is possible without Hemalurgy or Soulforgery.)
Step 4. At this point you should be in the same situation as Kaladin was in when Syl died, and you would need to repeat what he did. This means that you would need to live according to the ideals of the order the spren belongs to and swear the oaths.
Please leave a comment if you have improvements, especially with step 3.