I've also been really enjoying TLG, I'm finding it both very interesting and a lot of fun to play. Overall the main thing I've noticed is Eject Button is very strong in this format, probably as a result of the format being a fight to first KO. I'm not sure how we could go about balancing Eject Button for this mode if we wanted to or even if it's a good idea to balance it instead of leaving it as probably the strongest item in the format, just because if it can't be used for self-switching then I feel like that would greatly increase the importance of self-switching and trapping to the point where I suspect it would greatly restrict team selection.
With regard to Entei's Journey to the West, it does isolate 2 separate 1v1 fights, which I think was the intent of the arena, but more importantly, one of these fights will always have the same player ordering first, and the other fight will always have that player ordering second. In theory, I think a race like that to see who wins the fight where they have second order first could be interesting, but the problem is, the fights don't occur simultaneously; one of the fights always has its equal-number round happen before the other one (fight 1 round 1, fight 2 round 1, fight 1 round 2, fight 2 round 2, etc). This means that the person with second order in the first fight has a massive advantage because if both fights have the player with second order winning in the same number of rounds, which will not be at all uncommon, then the person with second order in the first fight wins because their KO round happens first. Thus, the person with second order in the first fight only has to KO in the same number of rounds as the opponent, but the person with second order in the second fight has to KO faster than their opponent. The one exception to this is if the person with second order in the second fight (i.e. the person with first order in the first fight) has a strong enough counterpick to the opponent's lead that they can win the first fight despite only ever having first order. I'm not entirely sure how to fix this, especially considering that the person with first order in the first fight is the only one who ever gets to choose a matchup, so any balancing that results in that person having second order in the second round of the first fight will then heavily favour that person, because then they both get to choose the matchup and have second order in what is often a KO round, with no other matchup control options available to the person ordering second in the first fight because self-switching, phasing, and trapping. I will note that in this case switching becomes an okay option depending on matchup, but if the opponent decides not to counterswitch then the player is still in a situation where both mons on the field have taken 2 rounds of stealth rocks, one round of damage, and they still have first order.
Overall I'm not sure how to fix it, because I don't see a way to lock in 2 separate 1v1 matchups without favouring one battler or the other, and if it ends up favouring the person with first order round 1, then it would never be picked because that player doesn't choose the arena.
As for bundle suggestions, based on the tierings of legendaries, the only ones I can see being approved are tapus (which there has been some speculation on discord that those are the next bundle), possibly genies (the highest tier genie is lower than the highest tier dog, and the second highest tier genie is lower than the second highest tier dog), and significantly less likely either galar birds or regis. Galar birds and regis would both have the highest average tier of all bundles so far, so that's why I think they're less likely, tapus would almost certainly would be approved because 3 of them are tier 3 and those 3 could be released in a bundle, and genies at least have a lower average tier than the dogs, although not by a lot. I think a Regieleki, Calyrex, Regidrago bundle is also a possibility, although I'm not sure how well those fit together thematically.