Since they already have the ATR72, which dominates the regional turboprop market globally, the Q400 is probably too similar.
Also, since it looks like the currently implemented gates are only Group III/Code C gates, widebodies are probably going to be best left for a much later alpha build. The A320 family could fit, as they split market share with the 737 and seeing one but not the other at an airport is a little odd. 757s likewise would need a slightly larger gate, and I'm assuming that for gameplay reasons they're treating code A-C as "small gates" and code D-F as "large gates."
The CRJs are being phased out lately, so I think the commercial niche that is missing is the new regional jets—the Embraer E-Jet series or the Bombardier C Series.
tl;dr: E-Jet/C Series, or A320 family.