I'm not really in agreement with the idea that Smash needs 3rd parties to survive. The entire reason it's worked so far is because they've been tasteful and very particular with the ones they've picked. Sonic, Mega Man, Pac Man, Ryu, and Cloud at the very least are gaming icons. Bayonetta is one in her own right, but I'm not sure she's necessarily up there in terms of pure star power.
Going forward, I think it's a little hasty to think a lot of indie characters are in, or 3rd parties in general. Clearly, conversations have led to the discussion of licensing these characters. This is certainly an important part of the process, and it's an important part that would likely only be endured for properties that Nintendo and Sakurai would feel are worth it. Thus far, I would say the ones we've gotten were worth the hassle.
Considering this game is slated for probably late or Q4 2018, any 3rd parties have probably already been decided upon. Crash N. Sane trilogy just got announced, and while I can't say for certain it seems a bit farfetched for that to be planned on Nintendo Switch around the beginning of development when N.Sane trilogy wasn't even announced for PS4. This is what makes me doubt Crash unless Sakurai believed the idea had merit prior to the Trilogy being announced for Switch. It's entirely possible, but I kinda think the hype around Crash currently is only because of the recent announcement, but that doesn't actually mean he has as strong of a chance as we all may think.
That doesn't mean I don't think he would be worthwhile. I just think the timing doesn't line up.
In terms of Nintendo "running out of ideas", I think it entirely depends on the character selection process this time. We already have Inklings, and there are a few other new IP not currently in Smash that they could choose from. ARMS and Dillon's Rolling Western are two examples of series that could provide a fun new character, though I'm sure at least Dillon would get pushback because he's "too niche" or something like that.
I think it's a good thing to keep in mind where additions and cuts may come from. While I don't want to see cuts, I think additions will be to series that are growing, while cuts would happen to series that are dormant. Now, I'm also a believer that the original 12 will never be cut (so Ness and Falcon at this point are safe), but if there were a cut , I could see it being Lucas unfortunately.
This is, additionally, why a series like Pokemon or Mario will always be in contention to get a newcomer from the fanbase side of things. Both series are continuing to grow all the time, and produce new and exciting ideas. When you compare that to the Mother series, which isn't going to be continuing anytime soon (unless Lucas gets a boost in recognition with a localization of Mother 3 which would be wonderful quite frankly), I could always see Lucas being on the chopping block, with there always being a good chance of a Pokemon.
Anyway, on the top of my head I can think of a good chunk of characters that could be added without, in my opinion, looking like it's the bottom of the barrel.
Decidueye
Ice Climbers
Rex (& Pyra)
Dillon
Wolf
Dixie Kong
K.Rool (I don't think he will be added, but I still don't think this would be a bottom barrel choice)
A Reworked Zelda (BOTW Zelda in place of TP Zelda)
Spring Man
Bandana Dee
Isabelle
Tom Nook
A practical Metroid character (Sylux or...a spoilery dude potentially) (this isn't to say Ridley isn't practical, but I personally am not entirely convinced, and I'm somebody who thinks a PRACTICALLY working Ridley would be great)
Now, things will be bottom barrel if we literally start adding characters that don't meet the "personality" aspect of Sakurai's criteria. Basically, not a character that can be mixed with thousands of others and not be found. In other words, different species from a game. This means Koopa Troopa, Space Pirate, Kremling/Tropical Freeze dudes, etc...The obvious exception to this is Pokemon, because it's Pokemon. The series is based on the different monsters, so it's different from taking a typical Mario enemy and putting them into Smash.
Overall, I think there's a point at which the game will be running out of good option, but we're not entirely there yet. But maybe that is when it should turn into a "game as a platform" kinda like Overwatch but not too much. If that makes any sense anyway.