Eliminating mandatory tyre changes would certainly liven things up.
It didn’t the last time it was used.
Pole sitter sets off in front, the rest trail behind in performance-order, controlling tyre use. Remember, only one tyre manufacturer, and it’s probable that they’d all be on the same tyre.
Any driver who tries to overtake will be extremely worried about whether they’ll have any rubber left with laps remaining.
While the cars have wings, the issue will persist. The BTCC until the mid-90s was brilliant. And then Alfa turned up with some aero bits and suddenly nose to tail but ten metres back was the order of the day. No overtaking.
Want variables?
I’d start with manual gearboxes. Proper ones. Nurse that baby around the track, game boy. No more millisecond gear changes, and plenty of potential for mistakes.
Let the teams/engine manufacturers use different engine configurations. There’s a budget cap anyway, so let them go for it.
Allow testing, as much as the teams want. Before and during the season. There’s a budget cap anyway, so let them go for it.