@bascb – I can only think of two solutions. The first would be to revive most of the original Long Beach circuit, which would bring it in to 3.72km, which is marginally shorter than Monaco:
The FIA might have issues with it, though. There was a disastrous A1GP race in Beijing a few years ago where the cars went down one side of a dual carriageway to a hairpin, turned around and then came back down the other lanes. It was found that the cars couldn’t get around the hairpin because it was too tight, but the important part to note is that the FIA decided that they didn’t like the practice of using both sides of a dual carraigeway. However, the roads in Long Beach are very wide, so the FIA may not have a problem with it.
The second solution would be to pave over parts of the concourse surrounding the fountain, smoothing it out into a single corner. By moving the start/finish line back half a lap, the drivers would be able to get into a proper running order in time for the corner, minimising the chance of an incident. However, this may be unpopular with the locals:
The real challenge for any circuit at Long Beach is finding a place to put pit lane. It works for Indycar, because Indycar is happen to have an open-air lane. But Formula 1 teams like the garages, and there’s no space for them. So they’d either have to do one race with an open-air lane, or maybe adopt the “demountable pod” concept put forward for that Santaner Londong Grand Prix thing.