Well, that is a bunch of questions wrapped up in less than 140 characters. I think I'll blow through a few paragraphs and see how many I can tackle without just listing answers.
How much bigger can grow an universe for a captions series?, do you think making over 5 to 10 stories is long enough? or you could make it neverending? Do you think making a neverending universe will get bored of it?
Universes have their own little quirks and foibles, and I think that how they are defined is what limits their ability to survive and expand. Something like the "Magic Taxi" which is a favorite of Martha can be limiting, since you need to have a taxi, and or a woman getting out of a car, then filling out the backstory wrapped around all the rules that were put forth. The Spells R' Us universe is much more open, since you mostly have an old dude in a bathrobe that sells items or helps people much in the manner of a trickster from various folklore and myths.
One of the quirks with those is it is pretty hard to expand them, since they were created with certain set rules and I'm not even sure you could get in touch with the people that created them. If you did expand them somehow, you'd be bound to have people that would complain that it shouldn't be part of the canon of the universe you are using.The advantage would be that most people have a decent understanding of the setup of your caption, and your caption wouldn't need much back story.
If you are talking more about a universe that was set up by you, then it would tend to be more flexible and likely for continuation however you want it to be, either by yourself, or inviting others in to play in your sandbox with the characters you've created. The broader the range of plot and people that populate the universe, the more likely you'll be able to keep generating new captions AND the likelihood that others will want to read what you've made and possibly make captions in that world too!
The ironic thing about the caption universe above (iBod) is that 3 different people independently had the same idea. There was a story on fictionmania, there was my take on the mp3 player (I am big time into music) and someone else on the Haven did one as well, without knowing of my previous work .. even weirder was that they made their iBod caption FOR ME! That speaks to the universality of the idea (a device that changes you) with a method that also transcends cultures and geographical boundaries (music) so I think the only limits on the number of captions made is how many pictures you can find of hot women wearing headphones and/or ear buds.
I think I've made about 12-15 captions involving the iBod technology. I can't remember the last time I used it so its been awhile. Was I bored with it? Not really, but it just hasn't hit me in the gut when reading people's preferences. Will I use it again? Well, considering that I can often be lazy and use pre-existing universes to spin TG yarns in caption form ... I would probably bet on it. In fact, I think I still have some "girls with ear buds" images on a hard drive somewhere.
What does all these things mean for others? I can't speak for the masses though I can offer up opinions for them anyway, I think each creator has a good feel beforehand how fertile their universe is by the ideas it inspires.That plus input from readers can rally help you get a feel for what can be done. Perhaps the universe has 3 incredible stories in it, and then it needs to shut down as a complete work. Maybe, you tell more, and then come back to it like an old friend. Or it could be such a useful story telling tool that people are using it 16 years later (SRU has slowed down, but still quite viable).
DISCUSSION QUESTION: For readers .. What are your favorite TG universes to see in caption format? For captioneers .. What universes have you created? Would you mind other people playing around in it? If you haven't made a caption in those universes in awhile, would you revisit it at some point?