stupid keyboard... anyway.

First thing ive thought about is to make tem have lvling potential in the same manner skills can level as we use them.

To better adapt the 4 basic forms, they need to match with specefic build types and receive bonus from legendary or unique items.

instead of having a gauge the fills up slowly. Allow the player to switch to apocalyptic form at any given time and that their lifetime can be either static to each invocation or allow the player to fill up guage before hand to have an Immortal Varriant of the apocalyptic form Which deals far more dmg and last less longer. Also benefiting from infinite energy / rage and 0 cast delay. Deeply increase atak speed, increased charge up time of all apocalyptic skills.

Later on when reaching lvl 50 Allowing the player to select a 2nd apocalyptic form which will have a Shared Skill added to the primary form. vice versa.
Also adding a powerful merge skill only useable when the apocalyptic form is in Immortal state.

Allowing to create new apocalyptic forms that can reach offer skills set that match better some classes that are lack luster with the current 4 forms.

by merging it would be possible to add 6 new immortal merged forms. (the shared skill would change before, but in the new form it would have its own skill set.

the merge forms would alloy players to chose 1 of thier duplicated skills to be attributed to the forms which will affect some variable prior to transformation. that can be that the form be Material focused or elemental or occult focused...Equipment will be able to be sacrified at a great cost of purple spheres to greatly affect the merge immortal forms..

This will not last just once.. if you which to mod it or change it you need to offer new equipment to replace old one. or you can add an entropy orb to keep rerol the stats of the equipment while affecting the appocalyptic form. Making the immortal form have higher chance of better boosts.

When killing monsters and dealing dmg in Immortal form it gives you a buff when leaving the form that boost your stats depending on the immortal form.
When in merged immortal form if you kill enough monster in the right time or deal enough damage without receiving excessive dmg..( dealed vs dealt ratio.) you gain extra time in appocalptic form.. when your immortal merged form runs out.. instead of returning normal you revert to basic apocalyptic form with a buff that can either boost your mouvment speed, cast speed, atak speed or retalitaion dmg.

After this form the appocalyptic form would have a red bar instead of the useall yellow one.. requiring you to fill the red bar with essences before being able to transform again at the upper basic transformation. Then if you want to accumulate to immortal forms you have to fill a 2nd bar...
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there is also the Games Megaman Starforce 2 Zerker/ninja or Zerker/ Saurian that inspired me a little bit.
Starforce 3 also have interresting noise forms. Red Joker is pretty bad ass... but thats another game.