Probably the same as now, G30 (.45) or the G21 (.45) but the longer Glock 21 is probably not worth extra discomfort/weight. I wouldn't change anything if I could still carry concealed since I would want the option to switch easily. Just uncover and recover.
You might get more answers from long experience on the OpenCarry.org forums
http://opencarry.mywowbb.com/ where people who do this every day have discussed this -- and continue to discuss it -- in depth.
Especially the forums about choosing equipment (obviously) or the VA, PA, and (maybe NC) OC forums which are (among) the most active participants in the movement to actually carry this way. Anyone in VA (legally allowed to possess the firearm of course) who is 18 or over can open carry without a license.
Even those with a CHL must reveal their firearm to enter a bar or RESTAURANT that sells alcohol, a bank too I believe -- or else disarm. This means that many CHL holders in VA is quite likely to open cary at least some of the time. Most of these choose equipment for CC and just untuck or or take off the cover garment when entering such areas. (Silly laws but we are used to this as reality.)
From my reading, and the issue I will have when traveling to VA (etc) if I wish to participate in OC, is the holster. Some form of retention holster is the order of the day for most people. IWB has a sort of inherent retention, but the folks that wear a full size handgun in an OWB tend to recommend the type the police are buying more frequently.
Retention holsters are a very involved subject -- almost like caliber selection or where to carry (OWB, IWB, shoulder, ankle, BTB, etc).