Grease is a very underwhelming spell as it currently stands due to its size and shape (10 ft square) and because the effect is easily overcome. I think this could be a great low level control spell that would still be useful even at higher levels, but it needs some changing. Control spells at 1st level that effect multiple targets typically have an additional or stronger effect: Earth Tremor deals damage, Entangle restrains the target instead of keeping them prone, among other spells having similar improved effects. See below for my alternative Grease spell.
1st level conjuration
Casting Time: 1 action
Range: 60 feet
Components: V S M (A bit of pork rind or butter)
Duration: 1 minute
Slick grease covers the ground in and turns it into difficult terrain for the duration. You make a line up to 5 feet wide, and 30 feet long; alternatively, you can make a square up to 15 feet across.
When the grease appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area, stands up from prone while in the area, or ends its turn there must also succeed on a Dexterity saving throw or fall prone.
The first change her is the spells area of effect. By changing the spell to be a line, this gives the caster the option to force enemies to cross its area; they may not be in the grease for as long, but they are much more likely to spend time in it. Increasing the size of the spell to 15 feet makes it to a creature moving through the terrain with average movement (30 feet) will have to spend all there movement to get from one side to the other.
The second change adds standing up from prone requiring a dexterity saving throw. This makes it much more difficult to overcome the prone feature of the spell, and may mean the effected creature needs to crawl out of the area in order to end its effect.
An additional change I considered, but probably would be too powerful for the spell being 1st level, was the grease being flammable. this is something that a lot of players ask for and DMs I’ve played with typically allow. Here’s my interpretation of how the effects would look:
A creature that falls prone in the spells area is soaked in grease: this makes the creature vulnerable to the next fire damage they take before the spell ends, unless they are already resistant or immune, in which case this has no effect. Any fire damage that is dealt to the grease area or a creature soaked in grease while still within the area causes the entire area to erupt in flame, cancelling out the prone effects of the spell. Any creature in the area must make a dexterity saving throw, taking 1d6 fire damage on a failure. The grease is completely burned up in 1 round.
I think this addition is a bit more complex than most things in 5e, and likely too strong for 1st level. The vulnerability to fire damage would be close to everyone in the area of the spell, which is why the damage is so small. I think boosting the spell to 2nd level be appropriate, and adding concentration to the spells duration.