I've discussed this topic quite a bit with friends previously so this is just word vomit from what i can remember from those conversations
The inaugural VGC smogon draft tour, Sitrus team tour (VGC draft team tour) and multiple DOU drafts has drawn in a huge number of doubles and VGC players to draft in recent months. I think it only makes sense to include atleast 1 VGC slot to represent this group of draft players.
In terms of other singles format, i think gen 6 or above is the best candidates as they're well developed formats with a big player pool and has been hosted for classics already. Gen 3 has picked up some steam in money tours and the TDT team tour so it could be a good candidate. I've seen gen 4 tours and know it's a format in the LPL team tour currently though i don't have enough first hand experience with it to comment too much, just wanted to note it as a potential slot as well. Gen 5 seems too underexplored to be considered as a stronf candidate though, only ever seen it in niche leagues and WPF regi division.
Lastly, i wanted to touch on natdex as a slot. There's too many differences between leagues and tours that have ran natdex in the past. The most recent draft team tour with natdex slot was PWS which had megas, no z moves, no tera. Many leagues now include tera, some with megas, some without megas but still without z moves. On smogon, i don't believe there's been a natdex draft tour so going off other natdex tours, it still include megas, tera and z moves. Additionally, many leagues are "non legacy" leagues that ban old gens only moves which as far as i know isn't a thing on these natdex tours. I think there will probably too many nuances and disagreement on how the format should work for natdex to be a slot for this tour.
The inaugural VGC smogon draft tour, Sitrus team tour (VGC draft team tour) and multiple DOU drafts has drawn in a huge number of doubles and VGC players to draft in recent months. I think it only makes sense to include atleast 1 VGC slot to represent this group of draft players.
In terms of other singles format, i think gen 6 or above is the best candidates as they're well developed formats with a big player pool and has been hosted for classics already. Gen 3 has picked up some steam in money tours and the TDT team tour so it could be a good candidate. I've seen gen 4 tours and know it's a format in the LPL team tour currently though i don't have enough first hand experience with it to comment too much, just wanted to note it as a potential slot as well. Gen 5 seems too underexplored to be considered as a stronf candidate though, only ever seen it in niche leagues and WPF regi division.
Lastly, i wanted to touch on natdex as a slot. There's too many differences between leagues and tours that have ran natdex in the past. The most recent draft team tour with natdex slot was PWS which had megas, no z moves, no tera. Many leagues now include tera, some with megas, some without megas but still without z moves. On smogon, i don't believe there's been a natdex draft tour so going off other natdex tours, it still include megas, tera and z moves. Additionally, many leagues are "non legacy" leagues that ban old gens only moves which as far as i know isn't a thing on these natdex tours. I think there will probably too many nuances and disagreement on how the format should work for natdex to be a slot for this tour.