I don't think there's ever a case of a model being announced less than 6 months prior to launch. The current system I think has a default production speed, based on RL/plane size, for each model. Which can then increase by a certain amount, depending on orders. This causes two issues. One, some larger models have lines too slow to be very practical. Two, some popular models sell out so quickly that if you only log-in 6 RL hours after the launch, you can be looking at 5 years of full production queues. The first issue helps cause the second, as it funnels more players towards a handful of models. And the second gives a huge benefit simply for being lucky enough to be online at the right time, which is one of my pet hates, and something I think should be avoided if possible.
So, my suggestion is this:
When a model is launched, accept orders for the first 6 months (2.5-3 RL days, enough time for everyone to log in) without issuing them production slots. After that 6 months, allocate the slots. If a plane is really unpopular, use the current default speed. If a plane is very popular, use the same maximum speed, based on size/era, regardless of type. Base it on the current maximum speed of popular lines. Say for V.Large/JA (707, DC-8, TU-114), starts at 10/month, steadily increasing to 20/month after 12 months. For large, modern (737NG, a320), starts at 20, increases to 40/month. If a plane is somewhat popular, use a speed that gives the biggest order roughly 1 plane/25 days for v.large, 1/20 days for large, 1/15 days for med/small.
Then allocate the orders, whoever got the launch discount gets the first plane, then earliest orders get earlier planes, and everyone who ordered gets their first plane before anyone gets their 2nd.
Instead of taking 12 months or more to get the first 7 planes because production is only 4-5/month, you know you'll get 7 planes every 5 or 6 months, unless more than 20 airlines have put in a biggish order. Instead of being forced to wait 5 years for your first plane, you know you'll get it within a couple of months of launch, and get a steady supply, same as everyone else.
One potential problem would be when an existing line gets a new model, like the VC10 just did. New slots appear, people could order the older model and lock those slots up. Simple solution would be that if potential new slots are added thanks to a new model, they are automatically shown in white, as 'reserved for Super VC10', and then allocated to actual airlines 6 months later.