I think everyone so far is in agreement that running out of challenges like what happened on Sun to the riders isn't the best rule and needs to be improved.
a) Challenge any time you have a timeout, lose the challenge, lose the timeout (no freebie)
b) Unlimited challenges, if you lose you either lose a timeout, or, if you don't have one, you are assessed a delay of game penalty.
c) A system like we have, with one freebie "challenge" timeout. So your first UNSUCCESSFUL challenge burns that before it attacks your timeout and then delay of game penalties.
I also think any call should be challengable. There have been a ton of calls that go bad that are "unchallengable". Along with that, I don't think challenges should be allowed to be withdrawn. If the challenge flag is thrown, either the ruling on the field is reviewed and overturned, or a timeout or delay of game penalty is assessed.
Video review is there so they get the call right. If it doesn't work, it's become a liability. Fix it or get rid of it, because right now, we also all seem to agree, it's broke.