Okay, I think there's a lot of attack vectors in your ideas that you maybe haven't thought about, so lets see if I can tackle them. :)
No usernames on posts
This is interesting, but as others have mention this takes away the social from the social media platform. Also, you will not remove the 'friend voting cabals' anyway, as people will DM each other saying "this is my post, upvote please". So at best, you remove the upvote bots. However, the expense comes at plagiarism and abusive behaviour being completely unpunished. In steemit, I believe reputation matters hugely, more than the little number beside the name. Author persona (even if anonymous) is a hugely important factor in a social medium.
So, albeit an interesting idea, it wouldn't be a site I would be interested in using.
Anonymous transactions
I agree with this, would be nice to have a way to hide account balances and send/receive anonymous amounts.
Limited distribution to a single account
Okay. Here's where you miss the point of sybil attacks. Limiting the valuation of an account just means the creation of a new account. It doesn't matter if the account is limited when I can create unlimited accounts freely and easily. It's also incredibly easy for all the accounts to vote together, so all you have done is 'make it take more time', which is actually false, as one could power down their main account and power up alts, or mine with many accounts, or buy and spread to many accounts, etc.
I simply don't see any use of limiting a single account due to the sybil problem.
Linear SP powerdown to account balance
Once again, missing sybil attack vector. If the powerdown was linear, at in your example, 400 STEEM per week, then I simply make 250 accounts with 400 Steem each. Due to the principles of the site, and making all my accounts vote together, there is no reason not to do this as well.
Then, I can power all the accounts down, and since its linear, get all of the 250x400 steem in ONE week, and immediately dump 100k steem on the market. This is worse than the current situation of 940 steem per week.
So again, there would be no reason not to use sybils here, and I can immediately dump all my steem whenever I choose with this.
And for an example of how easy it is to use sybils, there's clearly 100 intelligent people that already upvoted this comment!
Your comment is right on the money.