With both Travis Touchdown and Shovel Knight practically deconfirmed, I'd say Gunvolt stands a better chance as far as potential Indie candidates are concerned. Besides Shantae, I can't think of any other Indie fighter that could beat him to the punch.
I'm really looking forward to Gunvolt 3. If Gunvolt turns out to be one of the remaining four fighters (unlikely, I know), I think revealing him near the release of his third game would be quite fitting.
I consider Gunvolt to be THE underdog Smash character as of now.
Gunvolt has upsides that neither Reimu, Quote, and other indies don't have.
He doesn't have the large legacy Touhou has, but he need to use Fan-Patches and rely on fanwork in order to have a proper international presence (which i consider a death sentence for Reimu) so i think Gunvolt wins out despite being a far shorter and smaller franchise.
Unlike Quote, his series became an actual well............series, rather than a one-off that got ported over and over. Plus, there's no Drama regarding Inti Creates unlike whatever is happening at Nicalis (which i don't think disqualifies Quote, but it's still worth mentioning) so there's that as well.
I also think it being a Japanese company also helps, it's just much easier to negotiate with Japanese Companies than Western ones, another advantage Gunvolt has.
I also belive that Smaller characters are usually saved for the last reveals, so i can see him being the last character.