My take would be the Ion Cannon. It would be lots of dice, either double or triple damage, medium range, AP/Super AP
I too wouldn't mind losing Beams for something like this - as long as there were enough AD to overwhelm Interceptors (or just declare the Ion Cannon to have the Mini-Beam trait). I wouldn't even need Double Damage, as long as there were enough dice at high range.
Take the Primus Main Lasers: 4 AD with all the Beam Goodies, except Precise. On average against a 5+ Hull, that's around 6 hits at Double Damage - thus around 10/10 against Hull/Crew after Damage rolls, not counting crits. Potential is there, though, for 10-12 hits if you're lucky
An Ion Cannon Alternative would be to keep the same range, keep Super AP, make em "mini-Beam", but give 18-20 AD with no DD. Or maybe 15-16 AD, with twin Linked. The idea being to get around 12 Damage rolls, again yielding on average 10/10 damage. And again, higher damage if lucky, but not so much. The real luck would be in the extra number of Crit chances.
This would give less punch per hit, but more overall smackdown potential, and slightly increased chance of Crits without resorting to Precise (which doesn't fit fluff-wise). If you don't make the Ion Cannon mini-beams, then just up the AD even a tad more.
In the end, the goal should be to make a race's weapons feel both representative of the race, and distinct from other races. This would certainly give the Centauri a unique feel. Especially if they actually had a small edge somehow over Beams - as if the Centauri had used big lasers in the past, but switched over to a new weapons doctrine, thus allowing the lesser races (Earthers and Narn) to abscond with the "obsolete" technology.
Just some thoughts...