Astral has hit my concern dead on.
While the idea is conceptually interesting, I don’t think per round saving throw rolls will meaningfully reduce the amount of saving throws you’ll need to build to be survivable. Being hard cc’d for 1-2 rounds in a running fight is a death sentence, and has been for all 19 years I’ve been on this server, and it’s only gotten worse.
Even if the caster doesn’t have a +1, conduits or a warlock summon can burn a flat footed character down in 1-2 rounds. Gate might take a little longer, but it’s still not going to be a slow kill. Some of the undead streams have utterly devastating dps potential.
What balances all of these out in a PvP standpoint is that their AB tends to be mediocre, and players can quickly screenwipe (most of) them with wof. If a player gets crowd controlled, they’ll become flat footed (which will cost most people between 12 and 22 AC depending on their build) and unable to press the screenwipe button.
For those who remember it, before we changed spellcraft, mind fog was a favorite go-to, not just because it softened targets up for more aggressive willsave disables, but because a lot of characters with the automatic -1d6 will save penalty would fail against the 1 round daze, since mindfog did not respect bonuses to saves vs spells (spellcraft and a couple other sources). A lot of skilled wizard players got quite good at killing targets in that one round mind fog daze window with their conduits or with vampire stream undead, backed up by a couple casts of direct damage spells.
To say nothing of group fights, in which any alert source of damage will be keeping their eyes peeled for targets that get disabled. As a serial melee build player, one of my highest all time sources of pvp kills has been friendly SoV casts. The stun on that is 2 rounds, but it checks everyone in it once per round for a fairly long time. Put enough people in there without mind spell immunity and someone will eventually fail a save. That 2 round disable is more than enough to close the kill if they can’t pray out of it.
I’ll also point out that we had to severely nerf the original launch of wild mage, because it had 2-3 saveless cc spells in its surge table. These were very short duration spells (1d4 rounds). They were also guaranteed single target kills.
Finally, Arelith has seen a proliferation of abilities that are semi-infinite or on cooldown which have a primary effect (such as damage or a difficult to remove debuff) and a hard cc “rider” with a dc usually in the mid to high 30s, and a short (usually 1-3 round) duration. Think a lot of the new spells that hemo gets, a bunch of warlock features, and all the harbinger buttons. These are currently balanced around the understanding that the saves will be failed only rarely on a very bad roll, so the fact that they’re available infinitely or near infinitely isn’t too problematic. That changes if people start regularly dropping saves against these abilities.
Tl;dr version: even 1-2 rounds of hard cc is an intolerable outcome. The amount of saves required to be survivable in a fight will not go down with the introduction of Tenacity. These spells will still be very binary.
If you’re looking for a way to make save or lose spells less binary, I’d suggest looking at the other end of the range, and adding a debuff or damage effect to most of these spells if they are cast by a slot limited caster that is using a slot to cast it (so no sequencers or spellbound wands) which applies when the save is made, but not exceeded by some margin. For example, the hold line of spells could come with an ab or dexterity penalty that applies when the saving throw is made, but made by less than 10 (e.g., -10 AB, reduced by 1 per point by which the saving throw dc was exceeded, or -5 dex, reduced by 1 per 2 points the saving throw dc was exceeded).
