Matrix already bridges through to many protocols (IRC, Slack, XMPP, Gitter, Telegram etc) but does so using bridges which run (conceptually) server-side. But yes, in the hypothetical daemon model, the same generic Matrix-like RPC API could also abstract other protocols directly (or local stuff like SMS & PSTN calls), similar to Telepathy.
I haven’t played with D-Bus much myself, so all I’ve seen is the Telepathy guys implying that it introduced too much complexity. An alternate solution could be to have the local daemon API just be the Matrix HTTP API (but immediately converting that to other protocols if/when it makes sense). An a final extreme would be to have the local daemon be a full Matrix homeserver (albeit one set up for p2p federation, in future).