Radical but flexible solution

Let's face it. You can't please everyone, so why insist on keeping this inflexible approach? Some would just like a water sim, some would like a few islands and some would like to run a business or residential sim. And everything in between. Why should the Lindens dictate how we use our sims... why should the lindens take control over this?

You have given us the power to create anything our minds can imagine, and even included the avatar rendering cost option a few viewer iterations back to battle lag. You didn't enforce a rule upon us, you said: Let the community decide. And that is what I think you should do with the sims.

My suggestion:
Only sell full sims at the current rate, but look at them as servers instead. Servers that are able to run the sims the owner wants. Give the owner management tools to set up the sims he wants on his/her server. One option would be having one sim on the server and use it's full power to deal with that. 15000 prims and so on. But it should be possible to use the server to host a number of sims that the owner can set up as he pleases. The total number of prims can never exceed 15000 prims, and I suppose there would have to be some limitation to the number of sims running on one server. The management tools provided should give you options to monitor load in each sim, and set limitations to prims, avatars, cpu load, events and so on.

The beauty of this is that the owner is the one responsible for load balancing and to battle abuse of resources on the server. The Lindens do not need to be involved and are not responsible for it. Almost any kind of sim can be created to suit the needs of everyone.

And by god... you should be able to rent a sim from the server owner. The owner should not have to be the same as the payor. Again this is enforcing rules on the community. You will get your money in the end anyway.

To summarize:
Linden Labs should provides servers that the owners can set up sims on to suit their needs. Linden Labs need to provide management tools for this. The owner of the server will be the one responsible for load balancing of his/her sims. This is a much more flexible approach that having a number of fixed products with different rules that needs to be enforced.

This is as I see it much more in the spirit of SL.

The big problem is probably what to do with all current OpenSpace sims and I have no good answer to that. If it's possible, maybe save the entire sim in some way and set it up in a new place later.

Well its a good start but one reason someone buys an openspace sim is because it's cheaper, not so they can have more land necessarily...so that would be forcing people to pay the cost of a full prim sim no matter what instead of having the option to pay less for less prims but the same land.

edit : thinking about it there should be choices like..7500 prims for 150$ a month, 3750 for 75$ a month, 2500 for about 50$ a month...and put whatever AV/script limits (like 20AVs for 3750 like they already doing it, 35AV for 7500, and 10AV limit for the 2500 prim) then just adjust the script limits over time to see what fixes lag problems for each type (sure the AV limit would fix that though), and then you can split it up for however much land you want. (a limit depending on how many prims you are buying)

Just a thought.