Consider this scenario: Human (with Variant Human Traits) Fighter 1 with Magic Initiate feat (Warlock spell list), followed at later levels with Sorcerer 1 through 19.
The feat's language says ...
You learn two cantrips of your choice from that class’s [sic] spell list.
In addition, choose one 1st-level spell from that same list. You learn that spell and can cast it at its lowest level.
So, selecting cantrips that can be made more powerful by casting them using a higher level spell slot appears to be allowed, but not for the 1st-level spell.
Here's my question: At Sorcerer 1 through 19 you will get higher level Sorcerer spell slots. Can they be used to make that 1st-level Warlock spell more powerful? If not, and the issue is Warlock spell vs. Sorcerer spell slot, would it be allowed to work if Magic Initiate was made to use the Sorcerer spell list instead of the Warlock's?
The way i see it, you can only cast that 1st level spell once per day, as a first level spell. It works like a racial spell, except it comes from a feat. So it doesnt matter if you take a Sorcerer spell and take Sorcerer levels later, your 1st level feat spell can only be cast once per day.
And i havent read anywhere near close to all of the spells, but my first assumption for the cantrips would be the same. you have two cantrips, you can cast them when ever you want, but they are always cast at 0-level.
Interesting...
Thanks, cowleymen 
All cantrips are cast at 0-level, no matter how you learn them. But the effects of cantrips scale with your character level.
Pennarin wrote:
Pennarin wrote:
AndyDay wrote:
Honestly, I think the aquisition of 2 Cantrips (usable at-will!) makes the feat pretty rad, even without the Level 1 spell (regardless of if it's ruled to be 1/day or 1/day+Spellslots)
mellored wrote:
Here's my take on it.
Compare the language with the language on racial spellcasting:
"Once you reach 3rd level, you can cast...once per day. When you reach 5th level, you can cast...once per day."
For the tiefling's hellish rebuke it lists what level it is cast at, since level can matter (it doesn't matter for the other spells).
Note what it doesn't say: It doesn't say you learn the spell. It just says you can cast it once per day.
Compare to the feat:
"...choose one 1st-level spell from that same list. You learn that spell and can cast it at its lowest level. Once you cast it, you must finish a long rest before you can cast it again."
I think learn is the operative word here. Racial abilities don't teach you the spell. You simply have the innate ability to produce the effect once per day as specified. Magic Initiate actually teaches you the spell. You can therefore use it like any other 1st-level spell you know, which means you can cast it with any level spell slot that you have available to cast a 1st-level spell in. And, in addition to that, you have the ability to cast that particular spell once per day without expending a spell slot. For those without spell slots, this is the only way you can cast it at all.
The intent from the language isn't crystal clear. However, based on the usage of the word learn compared to racial spellcasting, I would submit that a good interpretation would allow anyone with spell slots to cast that spell with them, in addition to the one free 1st-level daily use.
I will say I'm a convert to that position. I didn't think that was what it meant until I examined the language, but now I'm convinced it's the best way to interpret it overall.