I don't think we'll be only seeing third-party characters from now on.
It makes sense why they started out with third-party characters:
1. They need to negotiate the rights beforehand, something which takes time, hence they need to be prioritized
2. Great marketing potential, drawing attention from players and media alike, helping the game to have exposure even after its release
The order of DLC characters released so far makes a lot of sense:
1. Mewtwo - Very popular character with the option of being a free download for those who got both versions.
2. Lucas - Popular veteran character announced alongside Mewtwo's release date that helps solidifying that Mewtwo isn't the only DLC character. The fact his announcement coincided with the opening of the fighter ballot is telling that fan favorite picks are important.
3. Roy and Ryu - Melee veteran and 3rd party newcomer. One is a favorite from Melee and another is a very popular gaming icon, both characters being quite unexpected. The timing of this announcement at E3 helped draw attention as well and if it weren't for the leaks, this reveal would have a lot more impact then it did.
4. Cloud - First character reveal in almost half a year and an unexpected iconic guest. Being announced along with the announcement of a special broadcast, it's good marketing and has made an impact on players and media. This was made specifically to hype players, so they're confident that the broadcast will live up to that impact.
As with all of those Smash Bros.-specific streams/Directs/broadcasts, I imagine there will be multiple popular characters revealed along with a slew of extra content added on top of that. The stakes are high with how things have gone recently.