Well I have in smoothbore a 20, 16, 12, and 11 gauges. I have a 50 cal. rifle .
I just wanted a little bigger bore. A very plain Jane of a rifle.
Since you have a 50 (and a smooth 62), I'd go 58. Make it real step up. 50/54 are too similar to my way of thinking. My plan is to own one of each tenth of inch before doubling up with "next door neighbors". IE: one .3x, 4x, 5x, 6x. Before having a 40
and a 45 or a 32
and a 36. "Significant differences" is my way of thinking here.
I do have a 54 and really like it, next for me is 40 and then a 30. And then a smoothie of 16-bore, which will give me a fat 60-class. And after that I can sort out what's missing, if anything. If I were a scatterblaster by nature I'd go bigger yet with a 10 or 12 bore for sure.
But will much more likely make a chunk gun, and be satisfied with the 16 for all my smoothie needs.