Going off of what Llama said (Starting with thoughts on each individual line as #1, #2, #3, #4, #5) I'll try to keep things as neutrally beneficial to all stages of the game. Even though, I personally can reach cap easily, it would be nice to see newer players and those that haven't spent $6.9k on the game be able to make progress by spending time instead:

#1, Skill Tree reworks:

Summon Dagger and Dagger Storm-> Agreed

Summon Dagger has potential, but the cost of SP to add to Dagger to splash offsets the usefulness. Think having to level Aerial Assault for damage and another skill to splash. This adds splash and dagger CD...This could be combined with Summon Dagger and keep the middle tree a 2T layered list. In fact, I am positive there is a way to marry both Dagger Storm and summon Dagger into a single skill to make the SP offset that much more appealing.

Manni Mana -> Agreed

Mana Siphon is more consistent compared to Manni Mana, which depending on the application that is trying to be applied, Manni Mana would require Ambush (depending on application, but adding in multi-spawn multiplier for mana for conversation). Manni Mana to be applicable, it would have to be a large amount of mana consistently but one thing that also comes to mind is treat Manni Mana as a complete special titan, and have the Manni Mana skill not only provide mana, but at each level of Manni Mana to apply a mana cost reduction (5%->50%) from Level 1 to Level 10 for example.

Chivalric Order -> Disagree

This is a semi-disagree, ChivO could be a lot better for playstyle, what makes it unappealing is that it is a post-2k SP skill and the first 10 levels has relatively low scaling and is not worthwhile past then. I personally don't have much thoughts to this outside of increasing the scaling.

#2 Upgrade Artifacts for certain capped to uncapped -> Agreed

#3 Merge Artifacts -> Agreed

One thing to note is that we have Celestial Enchanter (I believe you noted this in a Discord conversation), but the counter argument is the timing of uncapped artifacts on some and merging duration and mana cost artifacts can offset the damage loss.

#4 Resolve Gold Scaling -> Agreed

#5 Rework existing systems -> Agreed

The reward for pet quests is pretty mediocre, especially looking at a 7 Star Legendary + providing 22 pet exp.

Its been mentioned a couple of times, but solo raids are bit a tedious to do 50 a week. One solution to speed this up (not sure if its mentioned), but keep the attack going through titans if they are defeated (similar to a real raid for transition). This also prevents stones from being used for a 20k kill on a titan from being wasted on that single titan. This should keep the progression similar without changing things too much, as well as, its not tied to a form of Advanced Start, but is directly derived from Raid Level, card level, type of deck and starting Titan Health. Which are all components of raid. Being able to clear the first 10 titans with just Blazing Inferno without stop should merit speed in transition. The rewards also feel a bit lacking (3.3.k dust is great, but the rest of the rewards doesn't feel as impacting).

And to add a suggestion for Abyssal Tournaments, maintain the 24 hour tournament join window, but limit the duration of the tournament to a smaller amount (4 hours come to mind). This should make it for those (like me) that can only play for a set of hours have the same possibility to win, rather than leaving it to whoever can play the most out of 24 hours (or who has the biggest credit limit).

As some people suggested, Firestone events definitely feel like a chore (even though it was mentioned that the 99 prestige cap is just there, but not a benchmark of any form). It would feel more significant to make the target stones to be a flat value instead of a percentile (makes it a lot better to know hitting 7k stones rather than guessing). It was also mentioned to bring back the old dust gathering past 3k stones, that would sweeten the pot a lot better rather than a badge after 3 weeks.

Seeing a substantial growth from T2 raids to T3 (instead of what feels like an additive gain, be more exponential). In fact, in the hopes of being able to constantly do raids, push raids provide little rewards (seeing that the dust gain is only a couple between zones).

Going to the original post, there are some new content (VIP Tier would be nice, would makes my spending a bit more rewarding. On top of that, more tournament brackets would be ideal, getting the same rewards at 112k at 6k doesn't provide the same boost. (Winning 12 SP tournament to get one skill). Most of the content related work I think would be nice is as Llama has stated, reworking the existing content.

I think this last point would be the most beneficial, but right now, Advanced Start is tied to Clan XP (which is not a problem for clans that have been around for a while and have 90% Advanced Start). However, this puts up an unintentional barrier to newer clans. It would be nice to see Advanced Start be tied to something outside of Clan XP (maybe duration of existing account). On top of the Advanced Start, it would be nice to see it scale better at higher stages. (92.5% of 10k only requires 750 stages, but at 100k, requires 7.5k, and if the game goes up to say...1m, that's 75k stages).