The halfling's Lucky trait deals with the die roll (PHB, p. 28):
When you roll a 1 on an attack roll, ability check, or saving throw, you can reroll the die and must use the new roll.
And so does Advantage/Disadvantage, since it refers to the same trait (PHB, p. 173):
For example, if a halfling has advantage on an ability check and rolls a 1 and a 13, the halfling could use the Lucky trait to reroll the 1.
The Divination wizard's Portent feature, on the other hand, deals with the check as a whole:
You can replace any attack roll, saving throw, or ability check ... with one of these foretelling rolls.
And here comes the interesting part - the three emphasized terms are only mentioned in rules as actions, not numbers:
The description of attack rolls (PHB, p. 194):
To make an attack roll, roll a d20 and add the appropriate modifiers.
If the total of the roll plus modifiers equals or exceeds the
target’s Armor Class (AC), the attack hits.
Saving throws (PHB, p. 179):
To make a saving throw, roll a d20 and add the appropriate ability
modifier. [...]
A saving throw can be modified by a situational bonus or penalty [...]
[...] proficiency in a saving throw lets a character add his or her
proficiency bonus to saving throws made using a particular ability
score. [...]
And ability checks (PHB, p. 171):
To make an ability check, roll a d20 and add the relevant ability
modifier. As with other d20 rolls, apply bonuses and penalties, and
compare the total to the DC.
All three go the standard way:
- Roll a d20, get a number
- In case of (dis-)advantage: roll another d20, get a new number (or
keep the old)
- Add bonuses/penalties, get the final number
- Compare the final number with the AC/DC and get the final
success/failure answer
From the strict RAW reading of Portent, we have to replace steps 1–4 (the whole check) with a number (foretelling roll), which makes no sense, as the result has to be boolean – success/failure.
If we loosen the restrictions, the logical thing to say would be "In the context of Portent, attack roll/saving throw/ability check is the number to be compared with the AC/DC" (i.e. the result of Step 3 above). In this interpretation "You must choose to do so before the roll" means that the decision is made before Step 1 and therefore no actual die roll happens. The modifiers are applied to the foretold roll as normal. This interpretation is reinforced by an unofficial tweet from March 2015 by rules designer Jeremy Crawford:
The portent die is intended to replace a d20 roll only, not any modifiers applied to it.
This result is Rules As Intended as well; the whole point is that the character knows in advance what is going to happen.
I'm not actually sure this is balanced at all
This feat would be giving the primary ability of a 2nd level Divination Wizard to anyone who takes the feat. Normally, in order to access this, you'd need to dip two levels into Wizard.
Yes, you'd get the other fun Wizard stuff by dipping two levels, but this is really the One Thing that divination wizards get, and they don't get 'more' of them until end-game.
Another option
I could see a potential case of getting one Portent Die, and not two. But that is still pretty powerful and becomes a must-have feat for any caster and possibly others too (although Lucky could also be seen like this.) The ability to guarantee a failed or passed save is HUGE - even if it is just once/day.
The feat for divination wizards
This basically gives them their 14th level ability at potentially 4th level (or earlier if V. Human.) I don't know the designer intent behind not wanting to provide more, but as someone who briefly played a divination wizard, these portents are very powerful. Limiting their total use and number was clearly something the designers considered when developing the class. If I had a third one (or fourth after level 14), that would be AMAZING.
Best Answer
This is probably over-powered.
Normally, there's a 10% chance of a given portent roll portending a crit, split evenly between success and failure (ie., either a natural 1 or 20). ... with some fuzziness around using the portents for rolls that don't crit; my experience is that portents are used for those rolls rarely enough to not worry about them too much.
With the expanded range, there are 24 possible values, 6 of which are crits; the probability of portending a crit becomes a whopping 25% (6/24 = 1/4); moreover, the probability of portending a critical success becomes 16.66% (4/24 = 1/6)! The expanded crit range means that portending a critical success is over 3x more likely than normal, and over 50% more likely than portending a crit either way normally.
And, that's before considering the alterations it makes to 5e's bounded accuracy model. One of the design considerations for 5e is that it's hard to get bonuses to d20 rolls and AC that differ tremendously between characters (previous editions made it relatively easy to have a difference of 10 or more - and differences of 20 aren't terribly hard - in some of those areas). This is part of why items that grant bonuses to attacks are relatively rare compared to those that, say, grant advantage or allow a skill to be used in more situations (eg., instead of granting a bonus on a Dexterity (stealth) roll, an item might grant advantage on it or allow the wearer to make such a roll without needing cover). This alternate feature allows for up to a "+3" to any roll, and will grant some "bonus" 12.5% (3/24 = 1/8) of the time. Further, imposes a "-1 penalty" on an enemy 4.166% of the time (1/24).
Another way of looking at that is that it increases the average of a d20 from 10.5 to 12, letting portents add a "+1.5" bonus.
Simply increasing the range of values a d20 can take is potent, especially since the range is weighted towards the high end. This is a great boon for must-make saves or must-hit attacks, even more so than a standard portent.
The increased crit range is incredibly powerful, especially for forcing crits on attacks.
It's possible that the remainder of the class might make up for this power in other ways, though I remain skeptical. The linked subclass, as currently written, doesn't seem to do it, though. The alternate effects (eg., The Lovers conferring disadvantage on a CHA saving throw) are weighted towards boons (that is, the vast majority of the cards grant advantage), further increasing their utility. And, the other abilities of the subclass aren't weak, either.
I very much like the feel of this feature. The alternate effects of the fortune dice feel flavorful and are well-tied to the general tendency of bards to buff their allies. However, many of the effects can be replicated without fortune dice via providing assistance, limiting their utility. And, of course, they're pretty situational. I can definitely see where the urge to augment them comes from, and I don't disagree with that urge. But, I think that the expanded range of portent dice is too powerful. I don't know how to resolve that tension, though.
I'll note also that it's unclear whether the bard chooses whether the foretelling dice are roll-replacements or grant their other effect when they roll the dice or during the adventuring day. My reading suggests that they can choose later in the day, but I'd encourage clarifying that. My gut says that choosing when the dice are rolled means that they'll almost always be used as portent dice, since the other options are pretty situational.