Something I've been toying with in my head:
Monstrous Combination
Prerequisite: Must be a corporeal construct or a corporeal undead.
Benefit: If you're adjacent to an ally that has this feat and has the same amount of HD and creature type as you, if you simultaneously spend a move action, you can combine into a single stronger being, thereafter refered as the Union!
The Union uses the best base statistics from each of the combining members, including ability scores, natural armor, skills, base saves, DR, SR, fast healing, hardness, elemental resistances and regeneration. If they had both the exact same base statistic, increase it by 2. HD remains the same, but Max and current HP of the Union is instead added togheter. In the case of size category, instead the Union has the bigger size from among its combining members. If both combining members had the same size, then the Union is one size category larger. This doesn't grant any benefit or penalty to ability scores. Natural weapons of the same type use the best base damage, adapted to the new size. If they both dealt the same damage, increase it by another size category.
The Union otherwise gains all the special qualities and attacks from the base members, including SLAs, spells, maneuvers, feats and so on. It cannot gain an ability that provides the same effect more than once. For example, if each base member could gain Cha to saves, the Union only gains Cha to saves once, even if the base members had different names for those abilities.
Each round, the Union gets a full set of actions for each base member (only a standard and swift action on the first round tough). However, it cannot perform the same kind of action more than once per round except for basic movement. So for example it cannot perform two full attacks or cast two spells in the same round. It could full attack and cast one spell. If multiple players combined, each controls one of the set of actions.
Gear is automatically equipped and resized to the union, altough it doesn't benefit from extra item slots, it must choose to use the item slots from one of the base members. Equipment that would have no valid slot to fit in is simply absorbed and becomes inactive until the Union is ended.
A Monstrous Combination is unstable by nature. After 1d4+1 rounds, the Union must make a Cha check with DC 10+number of rounds the Union lasted to hold togheter. After it ends, the base members cannot combine again for 10 minutes. The Union can be ended previously as a move action. Either way the base members appear in squares previously filled by the union (or adjacent if there isn't enough space) and cannot act again this round. Damage is evenly distributed among the base members, and status effects randomly distributed.
Special:More than two creatures can form an Union, as long as they all meet the prerequisites. In the case of size category, you need double the base units for each bigger size. So for example three or two medium creatures would result in a Large Union, while four to seven would result in an Huge Union, and you would need at least eigh for a Gargantuan Union.
