Abadom
To be honest I would have liked if Philip reincarnated as an OC in the Metal Gear universe instead of a taking over an established character, especially one like Sam. Him being a boy from the slums with a dream of one day wielding the sword to defeat the cartels that destroyed his family, and Sam being like some sort of an idol/master, all ending in our boy becoming his own player in this world, and the successor to the Rodriguez style swordsmanship equipped with his own bad@ss sword; would have been cool.
I think the next class might be overlord, from the game overlord 2 (not the anime). The class would give access to magical abilities and the ability to summon "minions". If you are not familiar with the game, the "minions" are small imps that serve the overlord and are divided into brown (physical combatants, able to ride wolves), red (fire mages, able to ride salamanders), blue (healers/necromancers sub aquatic) and green (venom-using killers capable of setting up scratches). I admit they would be a little weak to the main universe, but as you said monstrous species can evolve, it would be interesting if they could too.