I found similar suggestions to what I was thinking here:

http://forum.codenameentertainment.com/?thread_id=11670 and http://forum.codenameentertainment.com/?thread_id=11565

Basically, I'd like to re-do some of the objectives that are locked out (finished T3), just for a change of requirements instead of always doing free plays. Don't really need any ruby rewards in my mind, but I liked some of the previous suggestions too.
Maybe once all obj. are completed through T3 (or T4 if those additions come about) in a campaign, they all could open again at T1 lvl, and people could use those for their free plays. Would add a bit of fun and change of pace options to the grind.
Thoughts?

On second thought, if you already finished a T3 or higher level then going back to T1 difficulty would be ridiculously easy. So I propose that once an objective is finished you may redo that level or any level lower for free play only benefits. This applies to all difficulty levels (T1, T2, T3) not just the final level.

There are certainly some fun ones I wouldn't mind replaying over and over again. Slow Descent comes to mind. I've actually contemplated failing that objective just so I could play it again when I wanted to.

I second the idea of no rewards for replaying an objective. 97% of all the objectives I've done are done with me being away from the game for the most part. And the ones I completed were really just completed to get them out of the way so I could get the 11th 'crusaders. With a couple million idols and now legendaries there is little difference between a FP and most objectives wall. I suspect that I will not do any more real world objectives for a while. I never liked the finality of completing objectives and I can only get 4 new epics from regular JC at the moment so the rubies on thier own are not enough incentive for me. Unlocking an objective as a FP alternative would certainly bring some fun back into doing objectives.

P.S.: I wouldn't mind redoing some of the event objectives too, for the same cost of a FP and the same or lower odds of a silver/jeweled event chest of course! There were definitely some cases where I was disappointed that T2 of events were completely different than T1.