OK, this one is a great idea but lacks content, naturally. As an old school tentacle fan, here are some suggestions of mine:
-Girl kidnapping: take example from Slave Maker 3: as a tentacle monster, you will have to kidnap girls and build base/nest defences as people will sometimes come looking for the girls, resulting in base loss (but you escape), game over, more girls or males to use/transform in case of victory, netori (you proceed to seed a girl right in front of the lover/husband that failed to save her from you);
-Tentacle nest competition: yours is not the only nest in constant need of girls, sometimes other tentacle beasts will try to steal your girls for themselves (you will have chances/choices/events to recover those same girls form them by stealing them back). Every girl you get has H stats (cum in mouth, births, bukkake, etc..., the more the better), that remain active/visible if they are kidnapped by other tentacles. Your tentacle nest will have an upgrade to get: chemical sonar: this upgrade will let you see what your kidnapped (by other tentacles) slave is doing at that moment (resting, birthing, being seeded, etc...), something like the magic mirror in the first Leane game (yes, this bit is for NTR lovers, also fully avoidable if you build your nest right and defend it when needed, here attack warning would be good, no matter what you are ip to at that moment (seeding, exploring, capturing, etc...).
-Girl purity/corruption/loyalty system: keep the girl pure, enjoy her reactions at the expense of increased chances of escape; slowly corrupt the girl (careful don't make it too grindy here) and enjoy her slow transformation to loyal slave/seedbed: once loyal she will birth more tentacles, that will be stronger that the usual ones (a rarity system could work here).
-Tentacle parasites: living clothing given only to the most loyal slaves, keeping her seeded, aroused and helping her fight when assigned to guard posts: these slaves will be much stronger than tentacle guards as they will demoralise human nest attackers!
-Tentacle nest vassalism: discover new tentacle nests by letting some of your girls be stolen by other tentacle beasts, stalk the kidnappers, attack them (different approach system here: stealth, assault, alpha dominance competition), recover your slaves and lay siege (attack areas of your enemy nest, seed their girls and proceed to new sectors of said nest).
Once the vicory is yours, place your own tentacles as vassals at the new location and be a king!