Polter I genuinely didn't mean to be alarmist so sorry if my post came across that way.
I do find it upsetting when multiple cats are lost. A friend has lost three cats on the same road, and has finally decided to keep her fourth cat as an indoor. My aunt has had two cats fatally injured by cars, one of whom had previously been hit by a car and survived. Then a third who was injured by a car and so far has survived. She continues to let her remaining cats outside and I just cannot see that as responsible. These are fairly quiet residential streets, not main roads. How many lives must be lost before attitudes change?
Italian No worries - it is your thread and you're free to ignore my concerns if you don't feel they apply 
Part of the problem is that although main roads are incredibly risky, quiet roads can often be even more dangerous for cats as they simply don't develop any road sense. A cat is less likely to run out into the path of a constant stream of traffic. My girl who I lost last year was a bit like your boy - she was an outdoor cat for the majority of her life and would often plonk herself in the middle of the quiet cul-de-sac and refuse to move for a car until they were very close/ beeped the horn.
I converted my cats to indoor only three years ago and it was hellish to begin with! They also escaped the cat flap but with time they adjusted and are completely happy and content with being mostly indoors now. They do have outdoor pens so they can get some fresh air. I love how much more of their personalities I get to see since they're indoor cats - lots of mad fits and playful behaviour that used to get burned off outside where I couldn't enjoy it. I also don't miss the anxiety of them not coming home/ going missing, I know they are always safely at home. Obviously you can't guarantee their safety completely but most of the risks like RTAs, diseases, accidental poisoning etc are eradicated for indoor cats.
I think with your new cat you'll have the huge advantage of them never experiencing a taste of the outdoors at your home, or possibly at all if you take on a kitten. They can't miss the freedom if they've never experienced it, so training them to be indoor only would be much easier than adapting your last boy was.
With all the cats in rescue at the moment I would definitely advocate getting another , I would just keep them indoor only/ restrict access to a cat proofed garden. It's not cruel and cats can fulfil all their natural behaviours as indoor pets.
WRT bunnies and cats, depends on the temperament of the rabbit. One of my bunnies chases the cats! Most of my other buns wouldn't be so bolshy. But they're never left alone together unattended for obvious reasons. Your best chance would be to get both at the same time so they're reared with each other. Rabbits make great pets btw, just as interactive as cats IMO.
Good luck whatever you decide!