For Willa Billie is technically the neutral form, and for Rose there is Rosel and Rosion, and Roos / Ross.
Other ideas…
Willa: Willow, Lilium, Winter, Willoughby, Wilder, Wesley, West, Wren, Lai, Wynne / Wynn, Willodean, London (the last one might be stretchy, Lilian, Wen, Wei, Lan, Lin / Lynn, Lee, Wenton, Camille
Rose: Ross, Red, Rory, Rowan / Rowen, Roisin, Shiloh, Shelby, Senna, Roux, Rho / Ro, Rue, Ronan / Ronen, Ronnie, Rossi / Rossie, Jamie (this might be stretchy), Ren, Rin, Penrose, Robin, Wildrose, Rhodes / Rhodis, Roscoe, Rosson / Roson, Rosamel, Melrose, Roselyn (Not Rosalyn), Rosen, Shannon, Shawn, Shane, Sydney / Sidney, Sky / Skye, Skylar / Skyler, Sage, Satchel, Rennie, Sailor, Story, Charlie, Sawyer, Spencer, Claude, Ray, Roy