I have been thinking this over as well. I think we may want the Ballista line to ugrade into the "Machine Gun" line. So like ...
Archer (4) -> Bellybowman (5) -> Oxybeles (6) -> Scorpion (7) -> Ballista (8) ->
? -> Ribauldequin (15) -> Organ Gun (20) -> Gatling Gun (25) -> Machine Gun (30) -> Trench Machine Gun (35) -> Modern Machine Gun (55) -> Unmanned Machine Gun (110)
Right now the Bombard is the connection, but like you said its unlike all of those. However we need some sort of early gunpowder siege unit to bridge the gap. Or alternatively a more advanced Ballista that exists during the first cannons. At the moment that's what the Heavy Crossbowman does. It exists when the Arquebus is around. And then merged back at the Musketman.
There may be one choice, the
Polybolos. It is a chain drive Ballista. In other words a "Repeating Ballista". However its was WAY ahead of its time. May be better off as a "Sandalpunk" unit. The down side is that it doesn't have as much power as the Torsion based siege engines. Thus its more of a side branch than a main branch.
In addition there was also the
Helepolis which is basically a siege tower filled with Ballistas on each level. Again this is very cool but more of a "Sandalpunk" style unit.