I don't expect my thoughts to gain much traction at all, but I'm making this post anyways to get the discourse started because I do think there's real merit to this idea, even if it isn't quite right for now.
The idea I'm talking about is that of the briefly-mentioned oldgens tour (something I'll continue to refer to as an OldGensLeague for brevity's sake, although that certainly isn't at all what we would have to call it).
To preface however, i'd like to share some brief thoughts on the main 2 other proposed formats, as a way of giving context for why I think that an OGL would be a preferrable alternative.
UMCL is not inherently different enough from UMPL, and the bulk of what I've seen as propositions for how to handle it / make it distinct is based around flex slots - something that is already slightly controversial due to their unorthodox nature. And in terms of mini-PLs, we already have UMFL which did a great job of including not only the much newer UMs, but also giving a chance to newer players who haven't made it to the big leagues. If we want a place to shine spotlights on overlooked metas and players, as well as test out new tournament ideas, UMFL is the place to do that, and idt a UMCL is warranted.
UMWC is not nearly as egregious as it might initially seem for reasons that have already been discussed, chiefly by Celeste, but I still think it's slightly less desirable than an OGL. Strategic merging of regions I think will almost certainly be necessary due to how relatively smaller most UMs are compared to officials. While entirely viable when done smartly, I think this dilutes the theme of WCs which prides itself on more concentrated regional unity and shared experiences of players. Furthermore, A UMWC would still just be centered around CG tiers, when I think that CG UMs already have more than enough attention.
So, why OGL?
Having oldgens serves to flesh out and diversify the experiences of a given tier. It's fun to discuss how concepts do or don't apply across generations, and it gives players options if they enjoy the underlying concept of a tier but don't entirely enjoy CG. Furthermore, gen9 saw the addition of a fair few UMs (UUbers, VGC UU, Monocolour); I can't entirely speak for VGC UU and Monocolour as I am less familiar with them but the vast majority of UUbers development has occurred in SV, and oldgens have only been a reletively recent endeavor; gen 10 is just around the corner and there are many suspicions that it'll even be announced a week from now, and it would be a shame if these SV metagames were to slowly lose activity and interest due to having 0 visibility outside of their tier communities and thus not really attracting newcomers. An OGL would provide a place for larger UM community members to have a chance to see the older generations and potentially gain interest.
Potential problems with an OGL:
Certain UMs have a disproportionate amount of viable oldgens. The biggest ones I have in mind is ZU and AG although, maybe others too. The flip side is that the newer UMs would have a disproportionate number of less viable oldgens. While again I can't entirely speak for VGC UU and Monocolour, I find it difficult to imagine that their oldgens scene is particularly fleshed out given the amount of time they've been around. Not even UUbers which has had a headstart on those two has a particularly fleshed out oldgens scene although UUbers has definitely been making good efforts in more recent times. An OGL would "make the rich richer" so to speak by boosting the visibility of UMs that are already riding strong. It would also force communities to choose 2ish oldgens to be played in tour, because it's obviously unviable to run every oldgen of every community - the number of slots would bloom semi-exponentially further than what we already have. This would promote the oldgens that are already most healthy at the cost of the oldgens that need visibility the most, although perhaps those metagames are issues for individual UM subforumPLs to deal with.
Do I think an OGL would be viable for 2026? No. 2027? Maybe? We'll see. My point is however that I think the idea has some real merit over the main options currently being considered for a second CA tour, even if the OGL is nowhere near being fleshed out enough. This may not be the exact place to discuss the logistics of an OGL given how theoretical it currently still is, but if not here then I'm not sure where else. I hope that people give this idea a real consideration as I think that UM oldgens are truly a delight and I hope that they receive more recognition.