First time poster here as well. I have not read tSM yet but have read a lot of the spoilers.
I really like the thought of the bond changing in some way to allow for the radiant to gain the spren's powers but would add that the spren will likely gain something from the 5th ideal as well. The connection/bond between the two of them has become so strong that they give each other access to their respective realms/powers. For example, Kaladin will be able to draw wind spren and access the surges while Syl will gain more grounding in physical realm and retain her knowledge gained about honor. The spren may also gain full access to the physical realm.
My big theory is that Brandon will use the ketek format for major character and plot arcs (including the ideals). That combined with the number 10 is so important in this series this lead me to think the fifth ideal is a red herring based on 1+2+3+4=10; four vows are the "completion" of the journey and the last is the acceptance of its end. I also think part of the purpose of the bond is to teach/heal the radiant and that once a radiant has dealt with their personal issue the bond is no longer needed. Ultimately, the radiant and the spren will come to an understanding that they no longer need each other and it is time to start a new journey. Maybe through a connection to each other in the spiritual realm? That way the connection is maintained but could allow for connection to another spren in physical/cognitive.
Just looking at Kaladin for an example.
1. Life before death, Strength before weakness, journey before destination - At this point the spren is awaiting the radiant to accept the journey. This is Kaladin stepping away from the chasm and choosing bridge 4
2. I will protect those who cannot protect themselves - Protecting in general
3. I will protect even those I hate, followed by a personalized message - This is the defining radiant vow IMO. It also points kaladin towards the law since his vow Is that he will protect those he hates, so long as it is right.
4. I accept that there will be those I cannot protect! - accepting that there are times you can't protect
5. Destination before journey, Weakness before strength, death before life. - At this point, the radiant is fully healed and the spren is waiting for the radiant to know they are okay on their own and ready to step away. I think for radiants to swear the fifth they will have to have a new goal or phase in their life. For kaladin, he will step away and become a surgeon. I see him being an Obi-Wan type character in Book 6-10.
The words will be "enjoying the destination is part of the journey or Choosing a destination is the start of new journey, I accept that to become strong at something I must first be weak, accept the death of false beliefs about self before life begins again anew"
Other crackpot theory is there might also be something with vows 1/3/5 being related to shards honour/odium(passion?)/cultivation and vows 2/4 being related to the order of knights.
On the recreance, it could have to do with breaking bonds and that the Radiants/spren both so strongly held to the belief that they were a sword. Both the radiant seeing themselves as a sword since all that the bond has allowed them to do was kill and the spren physically spending so much time as a sword. Once the bond was broken the spren's cognitive form is trapped within the physical world and physical form is stuck in cognitive world.