The Fire Steed, especially, is interesting to me because it should represent the very best of what a tamer can do for a mount (given the difficulty in finding it and in taming it) yet it also has a pack instinct you would lose at 4-5 slot...so that's kind of tricky.
What I hope they'll do is make a tiered system that does not only up base stats but also considers additional abilities. For example, a modern Fire Steed ought to have Fire Eater or something specific to its type. Ideally, there should be a reason to up a 2-slot to a 3-slot and keep it there for pack bonus versus having a 4-slot or 5-slot for tougher battles.
The introduction of slot upgrades (for lack of a better phrase at the moment) would also be a perfect opportunity to revamp worthless pets like the Skree, and maybe consider--one more time--to give us back silver steeds! Maybe they could be 40% physical 40% cold 20% energy as a counterpart to nightmares. But then silver steeds are a bit tricky, took, since Paladins ought to have their own mounts, but eh
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