I'd have to agree with the Challenger style stock (but with adjustable comb?) and no Maxigrip, Pete.
End block I'm not so sure. Some Gamos have that Mercury-esque sloping insert held in by the trigger unit securing bolt. I'd be quite happy with that but with the insert made of blued steel. This arrangement would allow for less trepidation on stripping / rebuilding and no worries over the indexing of the securing nut inside the old Mercury / Airsporter block. A shorter sloping section would allow for more orthodox scope rail positioning too, so no need for "reach back" mounts as some people have to use on the older guns. For it to become reality, the fewer changes from the Gamo design the more realistic in getting a gun to market, methinks?