Personal opinion: I don't think Mana is an effective resource as used in LoL.
And because of it, Energy has lost focus.
For the sake of comparison, look at DawnGate (until it shuts down on the 10th).
Most champions are limited only by cooldowns. The few that have secondary resource management, genuinely need it.
Moya: An AA'ing bruiser on Steroids with a spamable stun, slow, and one of the strongest ults in game.
Nissa: ADC with good AoE and an insane move speed/atk speed steroid.
Sakari: Uses an Ammo system for her stackable slow and her anivia wall (creates small chunks, not the full wall).
Fenmore and Viviyan both use health, and have really strong healing.
The Upcoming "weapon master" used a barrage system for her abilities that, otherwise, had no cooldown. (shame we'll never get to see her finished product.)
And Freya: . . . who had the Lee Sin Syndrome of having a kit with everything in it (including HARD CC) and a secondary resource that can be fully replenished twice in a single burst rotation... )Ya, that approach is just as toxic in other games. Who knew?)
Secondary resources could not be expanded (except health). (actually, I think there was a rune to increase energy regen that nobody ever used.)
Not to say Dawngate didn't have it's own problems (Lillian having insane base damage, hard scaling, %max health damage and defense shred on every skill). But compare the approach to LoL's approach.
Nearly every champion is mana gated by default. You can buy items to remove that gating.
Resourceless champions are usually designed around aggression and enjoy low cooldowns.
Energy champions can't expand their resources, but get it back so quickly that their only true gating are their cooldowns. (exception: Shen).
Personally. If Riot truly wants mana to act as a gating mechanism, they should treat it like energy. Nerf item-based mana and mana regen into the ground. Give hugely buff base mana and mana regen to compensate (and/or start handing out more return gimmicks). This should also make balance easier on you all around. Since you no longer have, say... Sona being balanced around the fact that she can spam all her abilities rapid-fire once she gets a chalice/tear. They play styles you want from champions can be maintained all game.
If you do want to item scale a champs resources (say, Anivia or Ryze), tie it to their AP. +5 mana for each point of AP or some such non-sense. This can also help to further separate champions with overlapping kits (Sona/Nami, Varus/Kog'maw, Morekiser/Morello)
You're already going in that direction with the regen changes. I vote you just take your foot off the break and see where this takes us.
Y'all can start down-voting me now.