Choices usually aren't as cut and dry as far as "most profitable" imo, a lot of people went for the 727 series and I bought tridents, I think I was one of the few to operate Tridents in this game and I was fairly successful with it.
Families are getting introduced soon which will allow more strategy as long as the price isn't negligible so for example, do you want to operate the A330 which leaves you with the A330-200 and A330-300 which gives you some flexibility or go with the MD11, does the MD11 give you additional options as far as range at the cost of blowing a family on one type or maybe the ER gets you there. Geography is also a factor, If you're operating from Australia for example, you're looking at long range where you're looking at fuel costs and range vs. if you're a European carrier you're gonna be looking at a lot shorter range planes with low turnaround costs where you have a lot more options.
You could run ATRs or Dash 8s which is kind of the mainstream option, but Embraer has some turboprops that might work for specific cases, and the F50 could work. In jets you could run the A320 series or 737 which is mainstream but you also have the MD80 series, Fokker jets, or you could maybe run CRJ1000s later which would mean you'd want to start with the CRJ700, or ERJs.
The other feature that's coming is negotiations so realistically, for more niche planes, airlines are going to be able to negotiate which adds a lot of possibilities there. I'm not sure if plane size categories for airport handling is a factor/is going to be a factor but that would also have an impact. This game seems to have a lot more viable countries than others I've played so I'm thinking overall you're going to be able to make more use of niche planes than usual.