Firstly I'd start with the tracks. There are many on the calendar that just need to go. I'd eliminate 90% of the tracks where it has been proven that that overtakes are rare. The remainder can be used on rotation for historical reasons.
Remove the massive run offs and introduce some kind of gravel trap in areas on certain tracks. Drivers cutt the track limits all to often at the moment, and there isn't enough of a dis-incentive not to.
I would also want to work with the teams and the tyre manufacture to make it so that the cars can run in all but monsoon conditions. That probably involves a change in suspension geometry, tyre sizes, and ride height.
I would open up the engine regulations but limit fuel to a certain figure depending on the track so say 100kg in Canada but only 85kg in Hungary (if it made the cut). There would be no fuel flow limitation.
I'd also open up the limits on batter power/usage too, to allow development in this area, since it is directly applicable to road cards and other craft.
In light of that, I would further restrict aero (to reduce spending there) specifically the wake of the cars, and work to introduce more mechanical grip to allow closer following.
I would also consider a somewhat controversial rule of forcing engine manufacturers to supply any team (maximum of four) that requests their engine. This would be rebadged where necessary (to avoid marketing issues) and would require 18 months notice. That way a manufacture would still benefit from producing a world beating engine, but only for two seasons, but allow for instance Redbull to request a rebadged Merc, or McLaren a rebadged Ferrari.
I'd also want any new developments in engine tech to be passed to customers at the next round of engine changes. With 4 engines a season that would allow the works team a five race advantage. This would help offset them getting upset about the rule above.
I'd abandon the two compound rule. Run what you 'brung'
I'd also want cars to be started under their own battery power. They can kind of be already as we've seen over the past year or two, but it will never work properly unless it's in the rules.
Donuts, stopping on track to collect flags, celebrations etc.. would all be allowed.
I'd consider moving qualifying and the race to the same day. For fans it seems daft to spread out the limited run time over three days, especially when not all tracks have support races and entertainment.
On that subject I would also want equalise the races in terms of time. somewhere between 1h45 and 2h. Monza at 1h15 is a bit of a joke under current regs. This is F1 not MotoGP
Ease restrictions on Super Licences. IIRC Sebastian Loeb was denied the chance to make an appearance in practice at Adu Dhabi despite being a 6 time WRC at the time. It was right he was denied it at the time, but I would like to encourage more wild cards and cameo appearances where possible.