Well making a Metrovania will be a lot more of work, and I didn't mean adding her permanently to SCV3, just add her in the demo so players will have a feeling of how a Legends remake will be, also the putting her together may be easier now due to the similar skills of the already programed characters, has I recall she got only a few spells.
So here how it could be done, first use some sprites to make a Sonia skin for Trevor, let all the whip moves the same, as for her magic attacks:
-For the whip fireballs use the weak infinite throwing knives of Grant.
-The first soul spell she gets is a time stop, so use the stop watch program just add a diferent icon in the item window.
-The second one is a life recovery spell, again just use the program for the laurels or the meat but apply it as a subweapon.
-Third one damages all enemies in the screen, use the program of the holy rain item crush of Sypha just change the graphical effect.
-The fourth one is a small shockwave that is throw forward in straight line, use the program of Trevor's triple knives, just change the knives's image.
-The fifth is a instant death spell that kills all enemies in the screen, use the program from the "Rosario" Item just apply it as a subweapon.
-Finally she had a spell that makes her invisible for a certain amount of time, I guess the same code for the "Medicine Jar" item will work, but I don't recall if there is already medicine jars programed in SCV3, I think I picked one while playng the demo but I'm not sure.
Of course it will still be quite some work, but far better than start from scratch.