The points you mention are definitely opportunities for Steemit Inc. to take notice of and make some improvements on the Steem blockchain level.

One recommendation that comes to mind is to enable beneficiary features for post payouts, and to make it so that the more a post earns, the higher the % beneficiary payout would be. That said beneficiary payout could get redistributed as beneficiary rewards to the author's followers/post upvoters/special Steem projects of author's choice/all Steemians. These are just some thoughts and suggestions.

To reduce Steem inflation, a portion of the beneficiary payouts could even get somehow redistributed back into the reward pool or burned (this should be possible without major whales having to downvote overly overvalued and/or spammy/abusive posts). To help with post spam, the initial automatic beneficiary payout % for each post over 4 within 24 hours would be increasingly higher on a linear or exponential scale.

And to help with direct message spam, I have an idea revolving around increasing the SBD from direct message transaction fees once a certain threshold is hit within a certain time frame.
For example, one solution would be to increase the min transaction fee by 0.001 for each 100 messages sent within the hour. The fee would be reduced back to 0.001 if the user waits [(number of messages sent within last 24 hours)/100] hour(s).

These are just some suggestions. Feel free to share yours.