ETOPS return 737
 

On a 180 ETOPS before your ETP, you have to do a 180 and return for some reason.
You just crossed over a waypoint, so there's is only a magenta line forward, noe behind you to gauge the offset. How do you try to offset 15NM, until you get a new clearance.

I was asking myself exactly the same question (same type, same 180min ETOPS) and came to the conclusion there's no "software" way to do it, the only chance is to act as an aviator (and navigator). Initially turn back in HDG SEL and follow (roughly) 15NM offset on the reciprocal magnetic track. If time permits and no new clearance can be obtained, just insert the route in reverse and then you can set 15NM offset and enjoy the help of LNAV. It won't take too long as there are usually not too many waypoints on such routes.

Any other opinion is appreciated

If it's just an ETPOPs turn back and not an emergency then organise the clearance prior to leaving track.
If it's worthy of deviating without a clearance then create your own navigational track as you see fit while making a pan or mayday call as appropriate. You can't set up a 15nm deviation from a route that you aren't cleared to operate on anyway so use common sense based on the route structure of airways around you.
The EEP is not a brick wall in the sky so if you are not in an emergency situation just fly straight past it while the turn back is being coordinated with ATC.
That's my opinion anyway.

Unless your ETOPS alternates happen to be directly on your route, then you would not be following it, offset or not. Having left the route "for some reason", you would be proceeding to your alternate, and obtaining clearance as and when possible.