A lot of interesting ideas:
I don't think using two slots is the way to go, it complicates the game and gui, and doesn't give much. What to do if you get an item but already have one has been a long debate, and was never really finally decided:
- Just get one new item, i.e. replace the old one (as currently being suggested).
- Always get the same item again.
- What we currently have.
I agree it's at least time to try something else. My suggestion: we implement the first approach, but also make a little extension: some items come in 'single' and 'multiple' versions. This would allow even more balancing of items, i.e. a kart in last place might have more change of getting a triple zipper etc. Any comments? We have some time before the 1.0 release to find the perfect behaviour

Re ghost karts on rescue: we can think of that at/after (probably) 7.2: I want to do support ghost karts (e.g. play against a recorded race of others, ...), and this would easily fit in with that.
Follow-the-leader time: can you suggest a simple formula? E.g. time till first removal is 30 + 1.5*(number of karts) or so? You can try to get a feel for different times by modifying stk_config.xml (follow-the-leader interval) - atm not depending on number of karts, but if you could let me know what might be a good value I'll add this.