I've finally made my mind and voted for FA-18.
In a market perspective, new content should be the seller.
So definitely it should be a new plane, and a bi-engines is better imho ;)
BUT, in parrallel, in order to improve the product, existing elements HAVE to be improved, things like comms, EM environement, flight models, etc... In fact, check things in Prov's list.
This won't be the selling point, but what will keep the community on product.
It's a 2-branches process : sell more with new content, keep community by reinforcing the existing one.
I wouldn't mind buying another addon containing new planes, if the level of implementation is on par with what FC seems to offer (will have to see it before ;), AND if overall product continues to be made better by improving existing element. Comms is a perfect example of what could be definitely improved in LOMAC.
EDIT : in other word, I think that putting one as the "main" task and everything else as "optionnal" is not a good option.