Viggen had some obvious advantages, such as STOL capacity (but no "rough ground" capacity), and some not so obvious such as the worlds best data link system (at least a generation ahead of everyone else) and a very pilot-friendly cockpit (minimum information to the pilot).
However, Viggen had to short range for the West German and Italian airforces, and no equipment for air refueling. It was adapted to the Swedish system of a lot of war bases (highways) with mobile supply teams, which lowered the need for range - but I doubt NATO would change their "a few big bases" strategy just to buy Viggen.
One possibility would be to make the original Swedish design slightly more ambitious Viggen with bigger fuel tanks and space to add a refueling probe. Then it would be more attractive for NATO.
But that doesn't change the fact that Viggen mainly was US hardware, assembled by Saab. No american president would allow his aircraft industry to loose such a big order to some swedes. IOTL a Swedish attempt to sell Viggen to India was blocked by the US.
Viggen, Draken, Ikv 91, Bv 206, Bkan 1a - Sweden manufactured weapons for use in Sweden, with swedish tactics. They wouldn't be as useful in protecting the Fulda Gap against several Guard Tank Armies.