I don't think Murumana needs this. It's already strong on a few champions that have it as a core item (Jayce, Yorick, Urgot, Blue Ez) and it's good that it is a niche item, though a while back there was talk about reworking the item (the community did not welcome this change), so they decided to scrap those changes for a while and instead buffed Manamune/Murumana a bit (Pickaxe in build path instead of 2 long swords, increased AD to 25 from 20). I kind of enjoy some of the idea of what they were going for though, there was just a few things about it that didn't work well with the champions that'd build it.
Their idea of reworking murumana was this:
[added] passive :: gains +10 bonus true damage (increased by 2% of maximum Mana if any number of azure stacks are active). also gain 1% attack speed for every stack of azure active.
[remade] active :: spend 60 mana to gain 10 stacks of azure, then doubles the current mana cost and stack gain of this active. azure has a fixed duration of 9 seconds, and when it expires this active resets its mana cost and stack gain and then goes on cooldown (30 second cooldown).
The awe passive (the one that grants AD) was intact, and here's how the active worked. The first activation gave you 10% attack speed and bonus true damage equal to your bonus from Awe, and all activations after increased only attack speed (2nd activation granting 20% attack speed bonus on top of the first, 3rd activation granting 40% attack speed bonus on top of the first two, etc). It was a good idea but the true damage didn't work well on champions other than ADC's that could potentially buy Murumana because they would buy lots of armor penetration.
I feel as though the concept was good (spend mana to get stacks which enhance your damage output), but they did it in a way that only benefited one party (basically Ezreal, Lucian, Sivir, maybe Koggles). A few changes that would have worked to make the changes work on either type of champion that would build it (I'm going to put in a modified version of the above box quote):
Mana Regen increased to 50% base from 25% (on both murumana and manamune).
[remade] active - Azure: Spend 20% of your current mana to gain bonus physical damage equal to 45% of mana spent (on-hit and on single target abilities that deal physical damage). Also gain 60% attack speed. Azure lasts 10 seconds, 60 second cooldown.
Basically I scrapped the passive to make the active more powerful and reliable (also removed abuse cases where your attack speed is absurdly high). True damage was also scrapped in favor of physical damage to make those that build armor penetration not feel left out. Damage is overall increased in comparison to now (at 2000 mana with none spent, active grants 180 physical damage for 10 seconds along with attack speed so you can make better use of it, costing 400 mana). The damage does not decrease over the duration of the buff allowing you to use your mana freely, though if you **activate **Azure at low mana you don't get quite as much damage from it.