Can't comment on guild suggestions, as I wouldn't know how possible most such suggestions would be. Though it'd generally be great to see the guild system expanded

5. Less time considering a "costume store" and more time on playability and bug items. stuf like that can be considered after you have a working product.
I can assure you luxury/QoL/cosmetic items are barely discussed yet

As you say, for the moment stability and basic fixes are the top priority.
Spectral Serpents - Unlike summon ember, this skill does not decrease your power pool but the serpents summoned are pretty useless even with minion bonuses on gear. The new skill, "Ethereal", would add spectral damage and phase attack strength to all attacks when toggled. Synergy - each SP adds a bonus to max power points while an elemental potency skill is toggled.
While I'm generally really fond of synergies, I personally find Serpents to be quite the amazing skill. Serpents and Tempest alone have been my go-to option for most of the game.
Edit: It turns out that +minion damage does affect them. +damage and the such definitely affect them as well, albeit possibly not as "cleanly" as other direct skills. They also crit, so there's that

Many have stated their liking of the addition of the Evoker's poison tree to the summoner class in Global as it gave them a more active role in combat. While I do agree, I feel that the bone tree (with the changes noted above) would be more suitable as it better fits the theme of necromancy and it provides more utility than the poison tree.
I do agree that the Bone tree would synergize well with Summoners, DF ones even more so. However, if DF allows spells to be cast while active, Venom Armor would probably be a godsent. Even more so if it's "repaired" into its earlier version, when it would deal some decent damage back on attackers.
As a player, I'd rather have them both. But balance-wise and class-wise, it would seriously blur some lines

If anything, Tempest would probably fit Summoners best. But that's like Guardians asking for SoJ, so probably a big no-no, that one.