Personally I think they should bring back refueling, if for no other reason than that F1 looks really stupid arguing that it's too dangerous to allow it when essentially every other top-level series has it. NASCAR, IndyCar, LeMans, etc.
Passing isn't the problem in F1, we have lots of passing these days. What we don't have is lots of passing for the lead, because generally most races have 1 team who substantially outperforms the others (often but not always Mercedes).
So how do you fix that? IMHO (and it's not a popular opinion), you leave the damn rules alone for a while. Changing stuff benefits the teams that can afford to throw a ton of resources at evaluating many different avenues of approach to find the new maximum performance compromises. If you leave them alone then the teams converge on the broad strokes and each newly-discovered improvement gets lesser and lesser, so the lap time benefit of each dollar spent on R&D goes down.
Otherwise:
- The rules for pit stops aren't broken, don't mess with them.
- If you want the drivers to go all out, you need to go back to the kinds of tires we had in the Ferrari/Schumacher era. Today's tires behave the way they do because the FIA asked Pirelli to make tires that degrade. Degradation means the tires won't do a full stint at maximum pace, thereby introducing a strategic/tactical element where they need to decide when best to use that pace. There were races where Michael Schumacher did 25 qualifying laps in a row, which just isn't possible today.
- Personally I'm not really a fan of the "you only get 3/4/whatever engines per season" rule. Presumably it reduces costs (although I haven't seen an accounting of how much), but it has had a couple of major side effects. First is that it placed a huge premium on reliability, which has meant that a lot fewer cars fall out of the race due to engine failure. This hurts the midfield teams because it reduces the chances of them scoring podiums when the top teams have an unlucky weekend. The second effect is the one where a popular driver gets a 10 or 20 grid spot penalty because of an engine failure, thus ruining his race weekend 2 weeks before they even get to practice session 1. (and no, you can't get rid of those grid spot penalties if you want there to be any teeth in the engine limit rule at all).
- I agree with alfadriver about slowing cars down by reducing downforce. We saw that a couple years ago when they got tired of seeing "lap record, Michael Schumacher in the Ferrari F2004" at every race (well, every race at a facility that we had in 2004) -- there was a conscious decision to bring down lap times by adding downforce back into the rules, so we could start setting new lap records.
- Renewable fuels is pointless. The 747s flying the team cargo to a single event uses way more fuel than all the cars put together all season.
- Opening up the engine formula, while superficially appealing, is the kind of rules change that will require MASSIVE amounts of cash to explore. It's the opposite of leaving the rules alone. A few years ago I would have argued for relaxing the "freeze" and "homologation" rules to allow the other manufacturers the chance to catch up with Mercedes, but that seems to have happened naturally. Ferrari is making more power, and the Honda and Renault engines seem to be pretty reasonable. I think this now falls into the "not broken, don't change it" category (so, of course, it's set to change again in 2021... sigh.)
- IME, preference for car aesthetics is really about what era of F1 you personally first started watching the races.