People who like creating multi expansion pack neighborhoods, especially the ones that use all available expansion packs, tend to run into this problem. I'm talking about having to build more and more lots, with less ans less space, thus making it harder and harder to fit everything on one map, including all the homes necessary for the sims.
Here are some suggestions for dealing with the problem.
1. Pretty much all worlds will have rabbit holes. All the careers, not counting the professions are unavailable without them. However a significant portion of map's land most likely will be used by them. Instead of single type rabbit holes like Sunset Valley's city hall, use hybrid rabbit holes like Bridgeport's city hall which is the police station, military base and city hall, all in one. Also, whenever possible place more than one rabbit hole on the same lot. You can also place rabbit holes on the same lots as some of your venues. For example someone designed a world with a warehouse rabbit hole on top of a dive bar. The dive bar worked perfectly. The only problem was that the warehouse map tag replaced the dive bar map tag, and you couldn't access the dive bar by clicking on the map tag. However, try to avoid, using more than one of the same rabbit hole on the same map. This takes up extra room, is only good for aesthetics for the most part, and can cause some serious problems if the sims working there have co workers and the map's size gets too large.
2. Make your lots as small as possible where you can still fit every necessary thing on them. If you want to design a huge city, try to avoid having too many lots bigger than 30 by 30. Download the apartment mailbox here.
Don't forget to thank Aynat134, and Bluefunk for making them available without us having to use a mod. That way you be able to make apartments on lots smaller than 30 by 36, the smallest apartment lot in Bridgeport.
3. You don't have to include everything. Maybe you want to make a small town. Than putting all nine of Late Night's bars/clubs in your town doesn't make that much sense. A small town wouldn't have 3 dive bars, 3 dance clubs, and 3 lounges. In fact if it is supposed to be a really small town, most likely it would only have a single dive bar at most and none of the others. Sure you won't be able to accomplish all the skill challenges associated with Mixology, and the band career won't work right, but maybe you don't care? Maybe its not part of your world's story. Plus you could still put in extra empty lots for extra ones and give them a "no visitors allowed" tag. Sure it won't reduce the amount of space you use, if you have to add the extra empty lots, however, it will mean that the sims that are already living in your town will still be more likely to visit the other other venues, and you might not have to place as many apartments or houses, so you could save space that way. If you choose to go this route and say create a haunted town, (now that supernatural is coming out) and the only late night venue you include is a vampire lounge, here's something to consider. Don't use any Late Night content on the other lots, and give us the vampire lounge as a separate download instead of including it with the map. The map will still be listed as requiring Late Night with just that one lot, and people who have Late Night might be looking for maps with all the bars/clubs, and might not be willing to go through the trouble of adding their own, especially if you didn't give them room to do so. Keep in mind that not everyone has Late Night, or any other expansion pack for that matter.
4. Since patch 1.36 has been released we can now make some venues double as Showtime performance venues.
Check out this list and see how you might be able to combine different venues.
New Late Night and Showtime lot options in patch 1.36
You can use this as a way to reduce the total number of community lots necessary for your world. Also with fewer community lots where visitors are allowed, sims will be more likely to visit the remaining ones. Note that there's a small problem, make that big problem with trying to make a hybrid, Dance Club/Live Show venue, involving a scheduling conflict. Hopefully it will be resolved soon.
7/16/12 Important update DOLLERDRMS1 has told me that she tried to get performer to perform at a combined pool/Coffee shop venue. It didn't work. Not sure how it works with Simport now. I shouldn't have been so quick to assume that simport would work with combined venues. Hopefully EA will fix this soon.
5. But what if you want to include everything and think your machine is powerful enough for a megalopolis? Say you have an incredibly powerful machine, that can handle a world with well over three hundred sims and have enough space on your map for every single venue in the game so far, and your determined to make such a world? Go for it. With more than three hundred sims on your map, you shouldn't have to worry about getting enough sims to visit venues. However, many if not most of the people playing the sims 3 won't have a machine powerful enough to run your world right now. It will probably crash the slowest machines capable of running the sims 3 before its even finished loading, and if it does load on those machines, the frame rate will suck. Perhaps if your populating your world via the saved game file method, you should create two saved games, one with more than 300 sims and one with less than 150 for lower end machines. However, with all those extra homes you will have to include, slower machines may not even be able to run your world if no sims are living there, especially since you'll have to include extra homes for people who want to add their own sims or sims someone else created. Do you really want to go through all that trouble creating a world that most simmers may not even be able to play. Maybe you could comprise and create a world that will have about 200 hundred sims living there. You should still have a separate saved game file for slower machines with less than 120 sims in it. the world won't be as bustling, but more people will be able to play with it, without it crashing.
I hope what I wrote here was helpful. If anyone else has any other suggestions feel free to add them.
6. Important update: Writin_Reg brought this to my attention. I had known about it before but I had forgotten to mention it. You should keep in mind that the Salon and Fire can't really be combined with other venues. I remember hearing also that there's an issue with these career venues having rabbit holes on the same lot, so avoid doing that. They also have to be given a specific lot designation to function properly. I'm not sure about the consignment store, however, but she says there are problems with trying to put consignment registers on other lots as well. 7/15/12: Today Conorsim just told me that he was able to combine these venues with Rabbit holes without a problem, but other simmers have still had issues with doing so.
7. One more thing I thought of, but forgot to mention. I like to create World Adventures Venues for my worlds. However, keep in mind that they are luxuries, since you will always be able to visit one of them when you travel to a particular destination. They're nice to have, however, they are not essential, since anyone with world Adventures can visit one, regardless of what map they're using as the home town. In that respect the Academy, Chinese Garden, Nectory, and Markets and Small markets are not like the venues included in other expansion packs. Also keep in mind that the relics register only sells rocks outside of the vacation destinations and the general store register won't sell anything on the home map. You can include World Adventures venues if you want, especially if you include tombs in your world, but if you don't mind traveling to visit say a nectory or learn kungfu, you don't need to include them.