I considered all the players not just the forwards and came up with 2 likely candidates: Giovinco and BWP. I can imagine Villa might have made it in past either of them but for the scandal, but I can also see how they might have put both in for the reasons I identified. I think Giovinco is a more likely replacement for the might-have-been Villa. I think MLS people really appreciate longevity and BWP became an MLS lifer once he arrived. Giovinco has the same 4 years as Villa, with fewer goals but more assists. His main edge is the MLS Cup, which is a team achievement. But it's all speculation.Yeah it's tough to see which of the forwards Villa beats out.
Dempsey and Donovan are both US legends, Donovan is miles ahead in both G and A and Dempsey played far longer. I think you could argue about Giovinco, but the fact that TFC won MLS Cup with Gio and we didn't with Villa is enough to keep Villa out. Keane had more G and A than Villa and won 3 MLS Cups, while Josef Martinez has the single-season goal record and an MLS Cup. Jaime Moreno was an early MLS legend, over 100 goals and 100 assists, won the Cup multiple times with DC. Wondo's another MLS legend, not so much a US legend but as the #1 career goal-scorer he's gonna have a spot on the list.
I think the only debatable entry against Villa is BWP. Yes he has more goals and assists, but he also played twice as long as Villa in MLS and the difference isn't that much (BWP: 125 goals, 36 assists in 8 seasons. Villa: 80 goals, 26 assists in 4 seasons). NYRB has won Supporter's Shield but not MLS Cup, so he doesn't have the cup win advantage. Still, it's a really close debate and I wouldn't be surprised if the recent scandal did play into it.