When a creature who can see or hear you makes a saving throw, ability check or attack, you can expend a 1st level spell slot to add +1d4 to the check as a reaction. If you use a higher level slot the die is improved: +1d6 for 2nd, +1d8 for 3rd, +1d10 for 4th and +1d12 for 5th. You may choose to do this after you see the roll total, but before you know the result of the roll.
Maybe gate the ability to discourage dips: Check at level 2, Attack at level 6, Saving throw at level 10.
It makes me a ranger fan. While I don't think there are really any bad classes, some classes or subclasses get screwed at low levels. The PHB level one abilities are iconically Ranger things, but they are also little more than ribbon abilities most of the time unless you build them knowing what enemies and terrains the campaign centers on. It thus spends the first level as just an objectively inferior version of the Fighter, which bothers me (not as much as the Sorcerer mostly sucking through the first tier or the Eldritch Knight and Arcane Trickster needing to either multiclass or wait towards level 7 to shine, but still). In any case, the UA Ranger options seem more consistently useful, and more versatile in terms of having thematically atypical rangers. I've been wanting to do a bounty hunter oriented Ranger or a Horizon Walker who is some sort of defender of the multiverse, and this seems to open up more space for thematic consistency.
But returning to the PHB, the subclasses, with one infamous exception, have always been pretty darned strong and many of the other strengths of the class are locked behind the spells. I think some people have always been turned off simply because they read over base class skills and see a lot more things that were thematic rather than mechanically strong and because the beastmaster is underwhelming.
Even with the new options I wish they'd let me take great weapon fighting as a fighting style. Come on, let us play Aragorn already.