[...] since almost all dapps uses this url scheme : category/@username/permlink
Actually there is lot of different url scheme now across apps:
https://steemit.com/{category}/@{username}/{permlink}
https://busy.org/@{username}/{permlink}
https://d.tube/#!/v/{username}/{permlink}
https://dmania.lol/post/{username}/{permlink}
etc...
It will be great if 3p apps know about others apps url scheme, it should not be a big deal to make a Gist within a JSON of every apps scheme so anyone can implement it.
I like this solution with both canonial and app, it give app more flexbility.
RE: Steem improvement proposal : Canonical links.