I'm looking at this from the perspective to filling a perceived weakness of question mark on the roster. For me the two biggest question marks are CB and LT.
The Rams have had some great CBs Like Ramsey, Gray, Irvin, Lane and Green, however my fist choice for CB would be Aeneas Williams.
He would be perfect fit in this defense. I think White's health remains a concern and that makes the CB spot as one of the biggest question marks on the team. Williams really had a big impact the Rams Defense in 2001 when he was signed (unfortunately he only had few good years left at the time). Williams in his prime would be my first choice. Note a healthy White could have a Williams-like impact buy I'm just not sold on his health.
A close second would be Orlando Pace. I like A Jackson as serviceable but Pace would be a significant upgrade to an already lethal offense.
Donald would be great too but IMO that may actually happen so it is not actually in the fantasy realm of this exercise.
So many great choices and I have not seen a bad suggestion yet.