Alright, assuming a carpender joins in Water 7, the Straw Hat Pirates are officially up to 8 crewmembers (after Usopp rejoins). That's just two away from Luffy's goal of 10, assuming my 1st grade math skills are still at their peak.
10-8=…2! Hurrah!
So the crew should (in theory that Luffy's goal of 10 is the final crew count) get the next two crewmates soon. Like in the next few arcs. However a lot of people are saying how the upcoming arcs should be major story progressing arcs. Like confrontations or revealations between characters we've seen once or twice, or not at all (Blackbeard, Ace, Buggy, Warlords). I have two theories concerning this.
The first is that we still have several smaller arcs to get through. For example the next few arcs follow the same past arcs patterns of the crew arriving on an island, and fighting some group of people (Arlong Pirates, Skypeia Priests, CP9, etc.). The same pattern continues with the crew arriving at an island and fighting some presence there, but the presence will be all new characters like in the past.
For example, the next arc could involve the crew going to the merman island that Kororo (fat alcoholic mermaid train lady from Water 7) told them about. Then there the crew faces off against some group of mermen or fishmen, and perhaps we see Jimbei a little bit.
The reason I suggest this is because it feels like during an arc where Ace and Blackbeard finally meet, or something along those lines, the new crew member won't be essential to the storyline. Like if during the Water 7 Arc, if Shanks and Whitebeard just showed up and started fighting, who would have cared about Franky? The meeting between those two would have taken the main focus of the arc. Oda could clearly work around this, but past arcs usually had the crew member being a big focus of the arc in someway.
This leads up into a final idea to go along with this theory. Since this theory would mean there would be two more smaller arcs, this pushes back all the storyline stuff we've all been so desperately been waiting for... into one SUPER ARC. Shanks, Whitebeard, Blackbeard, Buggy, Ace, the Warlords, the Emperors, Dragon, Aokiji, Smoker, the Admirals, Garp, all put into one insane arc. I know this may sound crazy, but thinking about it, it sounds like a good idea. Seriously, it would be so hectic, but awesome. This is all completely theory. I have little facts to back this up with, it's just an opinion.
As for the second theory, it's much simpler. A character from the past takes those spots. The first theory holds no grounds if a character whose already been explained shows up. This would also help relieve the problem of the last crewmates having to deal with that awkward catchup phase.
For example, perhaps the SH get a Marine to join them (Koby and Helmmepo come to mind). Or perhaps Bon Clay joins (He already considers himself a nakama. Perhaps now he'll just ride on their ship). Or other people like one of the Skypeia dudes, Paulie (assuming he doesn't join now), or Gin.
Now this is just my theory on the matter. As I stated, I have little facts to back this up whatsoever. Still, it's an idea I do feel strongly about. I know Oda is control of the story and could easily just write his way around all of this. Again, I'm just going on what I've noticed so far in the story, and what I believe. I'm just looking for other opinions, be them good or bad.