I do like the first idea here. One of the benefits of having a percentage decay, is that with a given mana income, your pool with soft cap at some number.

With a 5% decay rate (for example, the numbers don't really mean anything outside the context of spell costs), and 10 income, your mana pool will level off at 200, when decay = income. One of the nice things about this system is that it fixes the problem of having a long period of peacetime, where you can accumulate enough mana to make casting even high-level spells pretty much trivial.

The rest of your ideas I think are thoughtful, but not to my taste, personally. I was very pro-casting points at first, but after playing 1.1 a bit, I'm much more ambivalent.

So... yeah, I would very much like to see a moddable decay rate in the game.

Rather than even a soft cap on mana accumulation, I'd prefer to see the Enchantress magic system rich enough to make hoarding at least part of a viable strategy. The game needs strategic spells that take multiple turns to cast, with variable mana inputs that affect results and casting time. Things like item forging or attempting to dispel another sovereigns strategic spell.

Not exactly. What I'm hoping for is at least a handful of spells with open-ended mana costs, such that in certain sorts of late games you need to make decisions about whether to stockpile mana for reasons like: needing to recast a big spell quickly because it got dispelled, wanting to forge a Truly Ridiculous magic item, or wanting to be nearly certain that your attempt to disrupt another sovereign's big overland spell works.

For me, the game currently lacks any interesting tradeoffs when it comes to the magic system. I want unlimited mana accumulation because I want me & the AIs to have to consider when or if we stockpile and what to do if can detect that a rival is stockpiling heavily.

That's one of the Big Questions that's not yet been answered because content was a second-class citizen through the beta and  released versions so far (Enchantress looks like it will take a different approach...).

As the apparent Lone Defender of the Dearly Departed Essence Mechanics, I'd prefer to see mana as held by the sovereign. Imbued champions would be magically linked to their sovereign and the mana they could spend would flow through that connection, preferably at a rate determined by their restored Essence stats, and they might or might not also serve as mana sources for the sovereign.

Mana-related buildings other than shard shrines could be fun, as production boosters or support for imbued champions. The cap thing (hard or soft) just strikes me as a short-game/RTS perspective that I'd rather not see in an 'epic' TBS. A mana bank building just sounds cheesy (or at the very least kind of un-magical), unless it is perhaps at the heart of a sovereign's capital, not buildable, and not affected by the mundane economy. Could be fun to have different outcomes to conquering a capital depending on whether the attacker is prepared to deal with a massive mana release from the conquered mana storage structure. But even that would probably not be so much fun for me.