Offer a membership option that applies membership to all characters on a Jagex Account. I suggest a price of $15/month.
Only one account can be logged in at a time, with multilogs purchasable.
First multilog $5/month
Second multilog $4/month
Third and on multilog $3/month each
So, for $15 you would get p2p on all your characters (costs $12.50 on a single character now)
For $20 you would get p2p on all your characters and can log into two characters at once (current cost would be $25 to do this)
For $24 you would get p2p on all your characters and can log into three characters at once (current cost would be $37.50 to do this).
For the third multilog and each one after, add $3/month.
This would encourage those who would like to play on several different accounts but only want to pay for one membership to upgrade to a Jagex Account Membership subscription (more $ for Jagex). It would also give a discount to those who currently play more than one account simultaneously.
A benefit of a membership system like this is you wouldn't be limited to only playing this or that account the same time like it currently is. All your characters would have p2p, and you pick any of them you want to play on up to the number of multilogs you're paying for.