Just going to post my thoughts on all the different possible tiers people have suggested, mostly to organize them because at this point even I don't really know what I want. Also, ties are a necessity please make it six tiers thanks.
DOU: Well duh. At least 2, obviously, and I'd actually probably advocate for only 2 per week - in this case of 3, after 18 people have been slated for DOU, that's either going to leave the third game per week being not as high quality as the other two or the remainder of the tiers being practically side attractions. Keeping it at 2 leaves plenty of space for the talent to be spread out more effectively.
BW: I haven't had a ton of experience in BW, but what I have played in Fall Cup or occasional fun matches was hella fun. Plus, this would be a good spot for older players just now returning since they wouldn't have to learn a new meta, just get back into one they were already decent at. 2 slots would be my personal preference, but I could understand leaving it at one.
DUU: As one of the more experienced DUU players who is still very involved in the DOU community right now, I can advocate for this meta being a great option to include in DPL. The recent drops have made the meta quite balanced, and there's room for a lot of innovation and discovery, potentially getting more people into the meta. No way this deserves more than one slot though.
DUbers: This meta is kind of a wreck tbh. It's so setup based that games can frequently be over turn 1 or 2, which isn't very fun to watch. However, there's a case to be made for new adaptations that can be found in a higher-competition setting such as DPL (at least in comparison to the ladder), and I've seen a few players such as n10sit and Yellow Paint do pretty cool things with it. I worry a little bit about getting enough variety so that it doesn't feel like the same games are being played each week, but I could be persuaded into being in favor of this.
VGC16/15/14: Including a VGC format seems to be against the spirit of *Smogon Doubles* Premiere League, but I can't ignore the fact that there are many experienced players who would be able to make these games consistently high-quality matches.
Mono: MPL is still going on and has included Mono Dubs, I don't feel like this is a meta that needs to be represented twice. While it is interesting, it's just not a relevant enough meta with a large enough player base to bother including it.
DPPt DOU: The few games that I've played have been actually really fun, to the point where I'm considering taking up DPP OU for its mechanics of no team preview. There's plenty of room for exploration and it seems to be an inherently balanced tier, which is great. Since this is only available in Custom Games and has basically no exploration, I don't feel like it's ready to be included yet. Possibly next year, but including it this year seems too hasty.
Rand Dubs: Waaaay too matchup based to be considered competitive at all, and while Audio did a great job getting it set up, there are still plenty of movesets etc. that make some mons more detrimental than helpful to have on your team.
LC: Has anyone actually played this? I don't know if I've ever seen or heard of an LC Doubles game, but seeing as those are the two tiers in which I have the most experience, I'd certainly be interested to try it out.
AAA / STABmons / PiC etc: Basically in the same boat as Mono, except they have an even smaller player base and would have to be done in Custom Games. Including anything form of Doubles versions of OMs probably won't go well.
One more meta that I'd like to bring up simply for discussion is 2v2, probably in a bo3 format. I doubt it would be worth consider because I'm not sure how explored it is, but there's a ladder for it and it was metagame of the month once so I figured it was worth bringing up in case anyone had experience in it or could vouch for its inclusion.
Wildcard: I'm actually not against this idea as much as everyone else seems to be - it would be a decent way to include some of the lesser-known metas without having to drag them on for far more weeks than they deserve. DPP / DUbers / LC / Mono / STABmons / 2v2 could all be decent possibilities for a wildcard tier.
EDIT: Lol I forgot XY. This could actually make sense over the second BW slot, as other people are saying. I've never played it though, so I can't make an informed opinion.
I've managed to say a lot without actually saying much of anything at all or be helpful in any way, shape, or form; but I hope you got more out of this post than I did. Gun to my head if I had to pick tiers now, I'd say DOU / DOU / BW / BW / DUU / DUbers or Wildcard.
