Wohoooo

Alright, so good start and I saw a guy do this on the Red Dragon and now I'll do it here. I'm going to improve spellcasting since its ancient. 

In terms of the At Wil spell to use, probably use Rime's Binding Ice (3rd level) seeing its added effect does add more control for the ice/immobile theme. As opposed to direct splash from Ice Knife.

More of the same for control spells; Wall of Ice (8th level) or you can use Cone of Cold (8th level) for direct damage (for preference)

Now for scary DMs; Heres an idea: Give the Ancient Dragon access to either Mass Heal or Power Word Heal for that extra lean into being more friendly to mortals, which can be healed by it for it can use it to heal itself (which is most likely)

The Control Weather spell seems kind of pointless, as they already have the ability to cast it while they're in their lairs. Of course the dragon might want to cast it outside of its lair, but with these statblocks existing to simplify the game, it seems out of place.

Quote from Trevon07 >>

Alright, so good start and I saw a guy do this on the Red Dragon and now I'll do it here. I'm going to improve spellcasting since its ancient. 

In terms of the At Wil spell to use, probably use Rime's Binding Ice (3rd level) seeing its added effect does add more control for the ice/immobile theme. As opposed to direct splash from Ice Knife.

More of the same for control spells; Wall of Ice (8th level) or you can use Cone of Cold (8th level) for direct damage (for preference)

Now for scary DMs; Heres an idea: Give the Ancient Dragon access to either Mass Heal or Power Word Heal for that extra lean into being more friendly to mortals, which can be healed by it for it can use it to heal itself (which is most likely)

 A good set of spells, though the healing spells may be too much since its essentially a full heal on command. That's entering into the mythic territory.

Quote from Trevon07 >>

Alright, so good start and I saw a guy do this on the Red Dragon and now I'll do it here. I'm going to improve spellcasting since its ancient. 

In terms of the At Wil spell to use, probably use Rime's Binding Ice (3rd level) seeing its added effect does add more control for the ice/immobile theme. As opposed to direct splash from Ice Knife.

More of the same for control spells; Wall of Ice (8th level) or you can use Cone of Cold (8th level) for direct damage (for preference)

Now for scary DMs; Heres an idea: Give the Ancient Dragon access to either Mass Heal or Power Word Heal for that extra lean into being more friendly to mortals, which can be healed by it for it can use it to heal itself (which is most likely)

 Those are some good suggestions, but DMs should be careful not to inadvertently change the challenge rating of a monster. Rime's Binding Ice also reduces a target's speed to 0. That is extremely powerful for a high CR monster–particularly one that can fly. Here, the simple change of a level 1 spell to a level 3 spell alone likely results in a CR increase of 1 or 2. That's why DMs should always be careful when altering a monster's stat block (and the MM explains as much at the onset). Also, Cone of Cold and Wall of Ice conflict with this editions conventions. Cone of Cold is too similar to Cold Gale while Wall of Ice is likely unnecessary given the dragon's ability to fly, use of Ice Storm, and Chill.

As for self-healing spells like Mass Heal or Power Word Heal, they are not recommended for a number of reasons. First, these abilities simply provide the monster with more hit points, so the need to cast the spell can be avoided by simply increasing the monster's hit points (it is a convention in this edition of D&D to increase a monster's hit points and remove self-healing abilities to streamline combat and avoid redundancies). This allows the monster to take advantage of those extra hit points without needing to spend an action (or for certain monsters, a legendary action) to cast a spell that may ultimately even be counterspelled! Second, the changes to the dragon's hit points will massively increase the CR of this dragon. If your goal is a stronger silver dragon, you would be better served using the Silver Greatwyrm from Fizbans.

Speaking of Fizban's, Fizban's has an entire section dedicated to making dragons distinct and adding personalized abilities/features. I think you will find the options there will provide the flavor you're looking for without fundamentally changing this stat block. If your goal is to make a particular silver dragon into a spellcaster, then your suggestions could be more apt, but you still run into the issue of making a CR 23 creature substantially more powerful. All in all, you'll likely find the already-impressive stat block here more than sufficient to challenge your players while still giving them a chance to succeed.

The healing spells are basically just for sadistic DMs that just run brutal encounters so dont consider that one too much.

Seeing it is a CR 23 we can get away with difficult maneuvers seeing as cloud giant can incapicitate on command. So Binding Ice shouldnt be too bad in the grand scheme. It is at will but the range can be limited and these are optional add ons if you need increased difficulty so it being difficult to face is exactly what i was aiming for. 

I would like to point out the Wall of Ice and Cone of Cold are supposed to be in the 1/day category. Alot of builds are online so this is not a ride or die change its a change to keep up with players that need the extra difficulty. I would add this to mine without the players having broken builds to insinuate the dragons are the most powerful material plane creature there is (minus living apocalypse).

As for it being against/too similar to other editions thats not an issue I consider seeing as that not what this stat block is for. Its case by case so if it doesnt work with a hybrid game, dont use it

It is a mistake to brush over the impact of reducing a creature's speed to 0. If that creature is flying, it falls 500 ft. per round. So, the dragon could use this ability in two ways: isolate creatures on the ground and damage them from a safe distance or immobilize creatures midair, causing them to fall, take potentially massive damage (20d6), and be unable to participate in an aerial fight. If you're also planning on allowing the dragon to use Rime's Binding Ice as it can use Ice Knife with Multiattack, then that is all the more reason to not to, in my opinion.

I can understand your apprehension about putting much weight in a given edition's conventions, but most DMs would benefit from giving the tens of thousands of hours of testing and design that went into creating the game rules their due. I've been playing for a long time, and I've only grown more confident in recommending that DMs lean into official game design across the editions. That said, my recommendation was more precise than a simple warning against defying convention. Cone of Cold would not be a strong change because it is thematically redundant with Cold Gale (and to some extent Ice Storm). How that interacts with convention is that this edition has made a concerted effort to eliminate redundancies. I recommend against Wall of Ice on slightly different grounds: its primary benefits can already be achieved with the creature's stat block, so adding it simply provides another way of accomplishing the same things (but having to forgo other options, including Multiattack, to use it). So, while Wall of Ice may be redundant in some ways, it is the opportunity cost that concerns me.

This is primary addressed to DMs who will read these posts and perhaps not realize the full implications of these changes. All DMs are welcome to do whatever will make their game fun for their players, of course. But everyone should make informed decisions. I hope your players enjoy your creation!

I think the paralysing breath is missing the recharge. Seriously, its not even funny. Why would you need a more brutal encounter than that? All your players took resilient in Con?!

Yeah, they did that to all the metallics' unique breaths. I can understand detaching them from the straight elemental breaths, but removing the recharge is silly 

This is the most wonderful metal Dragon I have  ever seen.