@Naen  
Ah well it was worth a shot.

@BigOnionbean  
The horn just looked a bit awkward so I decided not to do it.

@Naen  
Yeah, kinda makes a bit more sense to go with Starlight as I do not take Marble for the kind of mare to get all angry. If anything she and Pinkie would be a bit of a hot flustered embarrassed mess whom might have random outbursts of excitement and happiness before retreating behind her mane.
 
Too bad this pone doesn’t have a horn. I figured that a unicorn/earth pony fusion would if anything have the muscular natural strength buff of an earth pony, but would also be able to dabble into magic.

@BigOnionbean  
I messed around with fusing their colours together, but I’ll do lineart first.  
This actually started as a Pinkie and Marble mix but I switched it to Starlight.

Wish this was in color that way we could see the blend of colors the two mares would bring into the folds of this new girl…
 
Otherwise it just kinda looks like a scrunchy face Pinkie with her mane all dapped up by Rarity.