First, I really like your simulator and I will probably buy it soon. The main reason for buying the sim is the oculus rift support, thank you for adding this, I love it!!!

Like with all simulators, I think the aerodynamic flight behaviour is not perfect compared to when you fly a real rc heli.

To improve your models you would need some real world test data. This test data could come from accelerator / GPS / … sensors found in every smartphone or flybarless module.
While flying the real rc helicopter this data could be logged and imported into your simulator to improve the aerodynamic flight behaviour models.

You could have 2 situations

If you work together with a Vbar vendor like Mikado everybody could make log files.
The user could do a calibration flight where he has to perform certain predefined flight patterns with his real rc heli.
eg
1 take off pitch up - pitch down 10 times -> would give you calibration data for lift versus pitch stick position.
2 make 10 piroeuttes -> would give you calibration data for piro rate
3 make 5 stationary rolls -> roll rate in relation to stick positions
4 make 5 stationary tumbles -> tumble rate in relation to stick positions
4 make 5 hurricanes
5 make 5 funnels
6 make 5 piro flips
etc

with all this data you could calibrate your models so that the users real rc heli flies almost exactly like in the sim.

You could also work together with some top pilots and heli manufacturers where you could put some really good sensors in the heli.
Let the top pilots fly complex manoeuvres and see if your aerodynamic model is correct or not for this type of heli.

In my opinion most simulators are not realistic with certain types of manoevers like
tick tocs
funnels
hurricanes
...

Ok that's good news, but have you really done the mapping for e.g. backwards inverted hurricane or a big funnel?
I meet of lot of RC heli pilots and many let me fly and setup their helicopter (I mainly setup & fly Mikado or Align with Vbar). I agree not all fly the same but I still have not seen any Sim that is close to those real rc helicopters.

Most sims have problems with:
High speed fast direction changes
hard tick tocs
fast backwards inverted hurricanes
funnels

Things that work really well in most sims including yours
piro flips
pitch up pitch down
rainbows

Well this is the kind of discussion I was trying to avoid ;-)
It is difficult to say with words what the problem exactly is. And I really like your sim, don't get me wrong, I just bought it this week.

In general I would say;
my real Vbar helicopters fly better, like on rails, in funnels, hurricanes, piro funnels, than with neXt.
my real Vbar helicopters fly worse, more unpredictable in hard manoeuvres like hard tick tock, high speed direction changes than with neXt.

Another problem is autorotations with neXt.

It would be a nice feature if I could go to the field with my Vbar helicopter, do a couple of autorotations, take the log file (GPS, stick position, RPM, orientation,...) and upload it into neXt to calibrate the autorotation module in neXt.

I could do the same with a helicopter of a friend who wants to learn autorotations, so his helicopter behaves the same in autorotations than in the neXt simulator. This would be a big help for students.

I would pay extra for such a feature.