Wish allows you to duplicate the effects of any spell L8 or lower. Does that include a lower-level spell, cast at L8? Say, as a Wizard, casting Heal at L8.
I'm leaning towards yes, but it wouldn't be the first time I assume the wrong answer...
From the Wish Spell:
"...is to duplicate the effect of any spell of 8th level or lower...." (PHB 288)
The important thing is the wording on spells that are permitted to be cast at higher levels. So lets go look at one.
"When you cast this spell using a spell slot of 2nd level or higher...."
That leads us in a bit of a conundrum. We're using a 9th level spell to cast a lower level spell. However the question of which slot is being used is something of significant concern.
I could see two equally valid interpretations:
The simplest interpretation is that the spell can only be cast at the level it's written for and that since no additional slot is burned, the spell cannot be cast at a higher level than it's written for.
The second, slightly more complicated argument would be that this is an additional 8th level slot and you can cast the spell at any level you want up to 8th level.
All things considered (particularly the spell, it's other potential effects and the fact that it's your sole 9th level spell for a day), I'd say that the second interpretation is far more likely to be the intended one and would be the one I'd use for my games.
SevenSidedDie's answer provides a third possibility that merits mentioning, though I'd have to ponder its balance before introducing it to my game. That would be that the effect of the spell is in fact 9th level. This makes a modicum of sense, the slot being consumed (and thus the slot ostensibly referred to in the upgrade text) is 9th level, so it may make sense for it to be 9th level.
My main concern with this is that it provides classes with Wish in their list to cast any spell as a 9th level spell once per day. That's not particularly game breaking when it would otherwise be an 8th level spell (using the maximum spell level cap).
This interpretation has the advantage of the consistent use of slot, but may or may not reflect design intent. Jeremy Crawford unofficially provides a wonderful insight (sarcasm) to this when asked:
Use "Wish" to duplicate lower level spell. Is it considered to have been cast as a 9th level slot?
I'd let the caster decide.
You're casting a spell using a 9th-level spell slot, the effect of which is to create the effect of another spell of your choosing (so long as that spell is chosen from the 8th-level-or-lower set of spells). It's only one spell you're casting — wish — with an effect that is selected at the time of casting. Thus, you're expending a 9th-level slot for this spell, and its effect, whatever that is, will use this 9th-level slot for any effect calculations.
Phrased another way, you're casting wish with a 9th-level spell slot, and it doesn't matter what its effect is or where its effect text comes from — you're casting using a 9th-level slot. When you resolve the effect of the wish, that's what matters.
This is not odd or particularly overpowered either: you can always choose to cast a lower-level spell using your 9th-level spell slot, if you like. The only advantage that this use of wish gives you is access to spells you don't already have available; pumping an extra spell-slot-level of power into a spell is something you could already do.
The important thing to note is that a spell cast at a higher level is a higher-level spell. The Basic Rules page 82 says this:
Casting a Spell at a Higher Level
When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level
Therefore, heal cast using an 8th-level slot is an 8th-level spell. So if you use wish to duplicate heal, you can choose if you cast a 6th-level heal or an 8th-level heal (or 7th).
Rules designer Crawford unofficially agrees in a November 2017 tweet, though he doesn't include his reasoning:
When Wish is used to duplicate a spell, can it be duplicated at a higher level? For example, a 7th level Banishing Smite to bypass Limited Magic Immunity.
A spell you duplicate with wish can be cast at a higher level, as long as that level is no higher than 8th.
As said by the one responder, it isn't that a lower level spell (or slot) is being used but rather the Wish spell (and a 9th level slot) is being used. That is, it is still a 9th level spell with a 9th level slot -- it's just that the effect (magical descriptor) of an 8th level or lower spell is being simulated. (Example: is used to simulate Fireball, then it is a Fireball spell cast at 9th level -- 8+6 d6 dmg. Whether or not a caster could choose to cast it with a lesser effect is the DM's call, but given the latitude of the spell it almost certainly should be per the player's choice.) Note, also, that Wish (per the additional qualities listed in the spell) could be used to simulate another 9th level spell... it just would invoke the draining effects listed and the 1/3 chance that the caster will lose the ability to cast Wish again. That, too, is a critical point of order. EDIT: lol (over a year later)... and, of course, the month after I posted this Crawford clarified "x" relative to the power levels involved. lol -- figures...