It is indeed foremost a legal issue, even when a new marketing strategy (encapsulating future licensing) under the ownership of Volkswagen will eventually, and hopefully (since most other Volkswagen brands are already present), change the exclusive deal with EA, the current agreement still stands (for as long as it's binding).
Or the sublicense to PD by EA could become, for whatever reason it now isn't, a reality when the current status quo continues.
In the meantime PD could make a Premium Ruf version, the most logical one would be the classic Yellowbird and/or BTR, as then they would have already modelled the classic aircooled 911 shape to be used, slightly altered, in the case a Porsche license/sublicense becomes a reality.
That shape could become the basis of endless 911 variants to maybe even dethrone the mightly Skyline...
The legal position Ruf (and Alpina) have got as manufacturer is extremely rare though and this is only possible I believe in Germany although I'm not sure. It doesn't therefor extend to many more tuner firms as these cars are still legally Porsche's I guess.
Instead of only looking at tuner firms who modify current Porsche models I'd like to suggest the no doubt controversial option of replica's.
I'm not talking about the glassfibre/polyester shells on Beetle mechanics which are fairly common but about high-class 356 Speedster/550 Spyder/908/etc. replica's using genuine Porsche parts also manufactured by seperate manufacturers with their own brand names (don't know the brands by heart but there are a select few which make accurate and pricey replica's).
This most likely won't happen though but it would be another option to include classic Porsche road/racecars alongside the classic and current Ruf line-up which in itself is limited regarding a lot of Porsche's iconic cars not represented by Ruf.
Like I said it most likely won't happen as it could further alienate Porsche, although adding, when legally possible that is, more tuning firms may have the same effect (as the relationship between Ruf and Porsche is close).