GolfHos

and if so, what is it?

i mean we had  'metal' drives, ultra cavity irons, huge moi mallet putters, advanced 'hybrids' , removable weight drivers,  removable weight irons, huge moi drivers,  huge moi removable weight drivers...

so what can/will be done next?

as long as they keep changing the rules there are no limits to where it can and will go. the USGA is definitely pandering to the equipment manufacturers at this point. basically anything goes. it really is becoming a joke. the latest being changeable shafts. the rule used to be "plain in shape" and "permanently affixed" now it's pretty much hit what you bring, and if you can't hit that, go to the trunk or worse cart and change it.

I think any further leaps will involve the aerodynamics of the ball. You can't make a hotter ball in terms of initial speed due to the USGA rules, but I suspect you can make one that has more lift/less drag throughout a greater range of RPMs.

I'd guess somewhere around 99.95% of all golfers will never taste a par round.  Therefore there's always room for a bull*feces* sales pitch about how the latest and greatest will elevate your game beyond your wildest dreams improvement.  Unfortunately those 0.05% guys will set a few course records and every other club will decide they need to lengthen to 7500 yards, stimp the greens at 11.5 and grow the rough.  How will it net out?  Jose McDuffer will never improve relative to par.  Rinse and repeat.

Meanwhile, Cypress Point doesn't change a damn thing and Ben Hogan's course record has been matched twice and beaten zero in 50 years.  Proving?  Exercise for the reader.

Under the current rules there probably isn't that much room to have a huge breakthrough in club design.

I think there may be some room for improvement in shafts with the different materials they can use. Maybe we can get to the point that you can get a custom shaft engineered to the correct specs to optimize your results with your given swing and clubhead of your choice.