Hey guys, finally updated the OP supporters list! Sorry it took so long.
Personally I am thrilled just to see SNK representation in Smash. I was starting to think it wouldn't happen due to SNK's current position as a niche company, but they're such an integral part of Japanese video and arcade game history. And while I would have preferred a Metal Slug character, I'm perfectly happy with Terry.
That said, Metal Slug hasn't been deconfirmed. It's possible that later down the road SNK gets to put another character in, and I would be very surprised if that character isn't Marco (or the whole crew as alts, which many have suggested).
While I understand the appeal of the idea of the Metal Slug characters riding the tank, I'm personally not on board. First because I think it would severely limit their possible special moves. But second because it doesn't reflect how Metal Slug actually plays, and I think that's something that Sakurai has tried to respect whenever possible. Still, I'd be more than happy to see Metal Slug represented in any way, shape or form.
Here's hoping for no Metal Slug Mii Fighter costumes when Terry drops!