I agree on the cannon bit. The only cannon merc that has any place endgame really is cannoneer and ONLY coz lvl85 smoke bomb is ridiculously hax in terms of -ACC. It is VERY noticable if you don't have a princess or prophet to counter the -ACC effect. You will miss like hell.
If I were to design a B class cannon, I'd give it a skill that allows it to have the function of an inventor's summon without the summon;
What I mean is, at the start of the turn, the cannoneer AUTOMATICALLY will randomly attack the opponent with a regular shot. This shot follows normal attack rules and is treated as a normal melee... However the kicker of the skill is that while it's an auto passive, it does NOT consume AP. This means that the merc can then regular attack AGAIN if it so chooses. If seth'd and assuming the merc attacks 5x on regular attack + a possible max of 3 attacks on the auto passive... That's 8 hits on turn. No matter what, that is extremely painful. Being B class he'd naturally have good attack power, good def and a high multi rate.
It'd also make up for cannon class types having generally low attack power as a whole, and also make it a viable merc in pve for attacking.
2nd skill i'd give it would be a targeted shot (like deadly shot) without the huge AP cost of deadly, but all of the power. It would class as a physical attack NOT magic, and will only hit one target. Can also hit flying.
Btw, a cannon main won the last titan.