Heavy Artics and Tri-Axle deckers dont appeal to many operators in the UK mainly because of there extra fuel needed and as you said unsuitability for some routes not to mention the maintence on there Fifth Wheel and the fact a normal 4x2 decker can be had with long wheels bases, worth a mention though is ADLs 12.8m e500 with rear axle steering, this could potentially work in Aberdeen? Like I've said with Euro6 RHDs being used in Hongkong now, they could easily be re-certified for UK use.
However if ADL had to develop some sort of Medium-Duty Artic, maybe a 200XLA(Extra Long Artic), I certainly think this would appeal more to operators in the UK who are faced with low bridges etc whilst gaining fuel efficiencies through a lighter vehicle, Cummins already have the 6.7ltr up to 300hp/1200nm which could certainly cope with a GVM 18t bendy.
The issue with Tri-axles in Aberdeen is the tight turns/weird angles of the road at RGU and in Bridge Of Don, not to mention because the fleet is only about 150 vehicles you need the vehicles to be versatile, for example the E500s are cleared for all routes while the artics are cleared for everything except the 19 and 20.
I'm not too sure if a 12.8M E500 with rear steering would manage the
turn at Balgownie (Streetview's wide angle camera lenses doesn't really show how narrow/tight it is and the weird angle it's at), often you get buses meet there. Thankfully with the Artics brilliant turning circle there's never really been any issues there except in the snow. The E500s get round fine too, although on occasion you can feel the body vibrate as the rear tyres scrub the surface.
In general the 1/2 is an awkward route to work around, you need the larger capacity of an artic to cope with loading's between St Machar and RGU, however the E500s are more suited to the long runs to Danestone/Ashwood. Personally i'd put Artics on the 1/1A with E500s on the 2.
Just for reference the turn
here at RGU would be another possible problem point for longer tri-axle deckers (loss of traction, narrow etc), again Streetview doesn't truly show how steep/tight it is. Less of an issue but the turning circle at Ashwood can't facilitate a rigid bus longer than 12M either.
Yes you'd probably by right regarding it being named something like Enviro200XLA (if it were to happen).