Some suggestions, including two with a Spanish flavour :
1. Attempt to recreate Mr Bowkett's feat of getting 12 ft lb out of a BSA Meteor, while keeping it shootable.
2. A similar conversion on an ASI Sniper or Paratrooper repeater - -maybe including a bullpup conversion with scope intermount designed to clear the magazine tube.
3. The piece de resistance - the ASI Sniper, a rarely seen recoilless break barrel air rifle operating on a sliding sledge basis like the RWS 54 and TX200SR that came after it. Surprisingly good, and power not a problem. Looks and finish a bit naff. Maybe a DIY military stock, or even another bullpup project. I know a shop where there is a tired but redeemable one for just 45 quid......
Adrian