A BLM proposal - Removing Sharpcast

[Lumadurin]

Being able to stack up three and then use one to reset Astral Fire would mean we'd be replacing Fire I with Fire IIIs more

Stacking them would be ill-advised unless you'd completely mapped out the fight prior though.
Since you'd be making a decision between damage (thunder) or movement (fire III) whenever you spent this resource, and holding onto stacks would often mean lost damage in the long-run.

There'd have to be a change somewhere to compensate...

All you'd really need to do is give Fire III and Thunder III small potency dips to compensate for the increased consistency.
So perhaps Fire III could go down to ~210, and Thunder III could go down to 35 per-tick.
Perhaps Thunder IV could eat a small potency dip too, but that only comes into play during trash mob pulls in dungeons, so it's less important really.

Are they still Free MP wise?

Correct.
The procs still have the same effect as ever, but they'd just dip into a shared pool of resources instead of being independent RNG.

The current iteration of BLM was made to function with predictable free movements resources every 60s. Maybe if they change the rotation itself in 5.0.

Of course not right NOW. Anyone making any kind of suggestion and expecting it to even be considered before like two patches down the line minimum is a fool.
But that said, BLM wouldn't require any adjustments to its rotation with this change. All that's happening is we get a predictable free resource every 30s, instead of a free resource every 60s + a completely random number of additional free movement resources (that are potentially worthless; if you get more than one Thundercloud proc between your Fire I and Blizzard IV, it's a wasted proc, and Firestarter is always a wasted proc when you don't have to move unless Convert is up).