My understanding of exactly how you plan to use TP and MP might have gotten lost in the rest of the discussion, but perhaps take a note from DNC and require that there be TP usage; perhaps the form drains both while active and using TP for a form's ability negates it—which then allows you to take another form.

Something like: You need 100% TP to activate a form. Higher levels of the job have a longer recast (say 5-10 minutes) ability to give you maybe 80-90%. Perhaps there are Merits to this effect, debate whether full Merits mean being able to activate a form at the beginning of battle or still being one melee (or throwing) hit away from it. People would quite likely /SAM.

In the active form, MP and TP are being drained, but as long as you don't miss a strike, you don't need to worry about TP. Any abilities in a form require MP, which is also being drained. Perhaps there's low level gear for reducing that, but that higher levels aren't completely free of the MP/TP drain—instead more gear toward max MP and maybe a Store TP ability with a tricky maneuver to using TP for an ability and keeping the form (perhaps /SAM is key to this).

(I'm a mage, so I'm not all that familiar with how abilities like "Store TP" work unless they're anything like "Sublimation".)

Using the TP, with "Form of the Cold Blooded" for example, would mean using "Baleful Gaze", losing the form. Since Baleful Gaze can almost guarantee winning the battle, it seems a fair trade.

As for the moment of activation and what animation there might be (yes, an icon—terribly useful with SCH, among others) a sort of ghost animation of the creature form in question coming up behind the adventurer, syncing with them and fading away.

As for the Beastmaster/Blue Mage cross that this seems to be (and it does, but no less than SCH is quite like RDM without melee capability and with—as is expected—use of unique spells) the story of the job's inception could easily be that of clans of mages and fighters coming together. Yes, the Immortals exist, but with some good writing, it wouldn't be difficult to bring it all together. Learning it could be the cutscenes to unlocking the job.

To further divide Shifter from BST and BLU, perhaps have more abilities per form (adjusting recast and the like, appropriately) explained by the fact that it's not a beast/plantoid/etc. using the abilities but an adventurer, that you have "learned" the spells, you just know them. Perhaps "Feral Magic" skill alters MP cost and somehow has an effect on TP drain and/or requirement for activation.

I haven't played D&D at all, so I can't comment on any similarity. lol