Not directly discussing the merit of the unbanning of any single card, I want to point out an interesting observation someone already made before.
Unbanning a card makes sense if it makes new archetypes or strategies viable (or brings back old strategies that have disappeared), especially now that the format seems healthy and there's no need to ban/unban something to balance things. Of course, then one should think of how do we want the format to look like. Making other combo enablers legal can be a dangerous move. Not only because of the possibility that one or more of the decks resulting from the unban could be broken. The sheer number of different combo strategies can be a problem for the format. By now we already have many different combo archetypes. To cite the most prominent:

- Storm Combo
- Charbelcher Combo
- Dream Halls
- Reanimator
- Painter/Grindstone
- Dredge (less and less combo, nowadays, but still worth to be mentioned)

Each deck requires different hate: grave hate stops dredge and reanimator - but does nothing to the other decks - canonist may stop Storm, Belcher and Dream Halls (the latter not completely) - but is dead against painter/stone or reanimator - needle is strong against belcher (that has a backup storm plan, though) and painter - but useless otherwise - etc.
Of course there's one piece of combo hate that hurts them all: Force. The massive amount of viable combo strategies can make non-blue decks struggling to fit all the hate they need or simply die to a deck they were unprepared to face. I don't think anybody wants a field divided into blue vs. combo. This is why I would be reluctant to unban cards like Hermit Druid or Time Spiral (not even mentioning stuff like Oath): they may or may not give rise to broken archetypes, but at the moment I think there's enough combo in the format.

On the other hand, control almost disappeared in the last months. I think it could be nice to give control players some new weapons to fight. I don't know if Mana Drain, LoA or Balance are broken or not (the first two are maybe already too expensive), but if I was to test some unbanning, I will begin from those kind of cards.