Isn't that already achievable with nTH packet based mangle rules (All be it rather CPU intensive).
That's not really "bonding" per-se; that's just flow-based load-balancing. To me, "bonding" means being able to aggregate multiple connections together in such a way that all bonded connections are usable
by a single flow.
You could use "/interface bonding", but it is proprietary to RouterOS and only works with ethernet-like interfaces. So the only way you could make use of it with PPP is by using BCP (dumping the PPP interfaces in a bridge, although come to think of it I'm not sure that the "bonding" interface allows you to specify bridges as slaves, and I'm almost sure it wouldn't allow you to directly specify the PPP client interfaces as slaves even though RouterOS PPP client supports BCP), or by bonding multiple ethernet interfaces together on both ends and then running a single PPP client session over the bonded interface.
Strangely, the only non-proprietary, PPP-specific way of achieving bonding -- multilink PPP (MLPPP) -- is supported by the PPP client on RouterOS, but not by the PPP server. If your PPP client device is running RouterOS and your provider is running a PPP concentrator that supports MLPPP, though, that could certainly work.
Turning this back into a "feature request," though, I would phrase it as "please add MLPPP support to the PPP server in RouterOS."
-- Nathan