In theory, cimetidine could increase (potentiate) the effects of oral 7-hydroxymitragynine by inhibiting its metabolism, but in practice, I doubt it will have any noticeable effects.
Nothing will significantly increase the effects. At least to the extent that I know you are looking for. As mentioned above, taking plain kratom may increase the effects. But 7-hydroxymitragynine raises one tolerance tremendously, so if you are taking say 5 smokeshop sourced Hydroxie brand 15mg 7-hydroxymitragynine -- a total of 75mg of 7-hydroxymitragynine-- then there is nothing that will potentiate it to the extent that I imagine you want. You can take things that have additive effects, like Black Seed oil (get the capsules, they sell it on amazon (i like this brand:
https://a.co/d/4vccyUe), which can help increase the duration between doses (i.e., the amount of time before you feel like you need another dose).
But unfortunately there is nothing that will do what I know you're hoping for.
Also, like other partial agonists, I feel like 7-hydroxymitragynine has a ceiling dose (i.e., a high enough dose after which point it doesn't seem to do more). Not sure where that point is exactly, but I feel like it's fairly low, like around 100mg-150mg or so. Doses above, for arguements sake, say, 150mg, do last longer, and cause more side-effects, but they don't really get you higher. And the tolerance builds super fast.
For example, after getting some high strength 7-OH powder (like 80%+ 7-hydroxymitragynine powder) I've taken 150mg-200mg for my first dose and it got me quite high, even nodding a bit, but additional doses begin to fail to produce the same effect, even if they are larger. After using large doses for a day or two, I've taken single doses as large as 500mg, maybe more, which were surprisingly underwhelming and got me less high than that initial 150mg-200mg dose, while causing more side-effects.
In an event, there definitely seems to be a dose above which there is no additional high. And tolerance to the effects grows very fast. The only way to appreciate the effects of 7-OH is to take breaks.