I'm not sure why TP and random actions would necessarily be related though? Even without a TP mechanic, whereupon a boss spends its 1000+ TP on any of random abilities, you can have a set of abilities, each with different cooldowns shared with the others on activation, and each time the boss cools down he choose another at random. And if bosses could de-pace (and therefore to a sense de-script) their actions, you'd run into severe issues with the current tank CD model. Why should a boss bother attacking normally with its skills if it could just save up for a whole minute, and then hit you so hard that you need HG/HG/LD to possibly survive? In either case it's dependent on the enemy's AI to consider whether or not it should bank its resource. That AI would be only thing breaking scripting directly, but without a sense of internal rotations there's nothing for the boss to do but run the tank's day by, say, using only burst when it pops -flat damage or using only machine-gun attacks when it pops 'for X hits' mitigation, etc. Which certainly wouldn't provide interesting gameplay. The primary difference I can see from using a TP system is that a boss can then accelerate output based on damage dealt. In other words, the more you fail to mitigate, the worse it gets. That can be especially punishing for certain strats and compositions. I wouldn't mind seeing that on some fights or on some mob types, and likely therein taken to the extreme because that can be really flavorful, and create some compositions you don't see elsewhere (like a half tank party being not at all a joke) but it's not a system I'd personally like to see made universal (much like I wouldn't want to see such strange comps necessarily replace the standard 2/2/4 in but a couple select fights).
To review and summarize, AI is the core component, not the backing system or weights, which can be adjusted per mob. You can have different mob personality types that can generate any combination of adjusted TP (or none) and/or enmity from various sources and conditions. You can have a set rotation, an randomized rotation on a single shared cooldown, use two tiers of shared cooldown, or more. You can have combo systems. You can have weights of estimated actual damage vs. base potency that may cause a boss to consider whether its wants to continue its established combo, which the tank has set matching mitigation against, or abandon it and its bonus for something that's not being mitigated against. They could take additional enmity from healing done, mitigation done, damage done, or even... damage taken (fixating on something they've wounded, more and more - such that you'd need to sacrifice some amount of mitigation > health to secure future mitigation when it really matters), or even upon "thwarted" actions (its AI decided it should kill X player, and that healing or that covering mitigation prevented it... Now I really hate that guy.) Enmity might be abandoned entirely at points for a tactical decision. It can check for weakened targets every so often or under certain conditions, or every N enmity or TP gained. So on and so forth. And that's not even getting to positioning, attacking vs. retreating, coverage, suppression, and so forth. It doesn't have to feel like you're hitting striking dummies who have periodic attacks and stances. It can feel like you're playing a chess setting one too high for you (at least until you learn to bait it).