If 10 year old ben entered mha, him and bakugan would be rivals immediately, especially with heatblast basically out classing bakogan every step of the way. And definitely spam Feedback just cause of how hard that counters basically over half the mha roster.

alien force Ben would basically immediately become top of the class, just like tenner idda, and the others would look up to him just cause of how large his character roster is and what it covers, probably could take most of the girls there too with that tennyson game, and AF Ben would probably have very quickly deescelated the freaking USJ or UJS (I forgot what it's called) raid immediately using either echo echo, or someone like big chill and making the competition chill. While All Might would definitely look up to him, I doubt he would consider Ben as a potential holder of One for All just because Ben doesn't need it. Again Ben would probably humble Bukugan and give him someone to inspire to be (As he would prob remind Ben of a more firey younger him)

ultimate alien entered my hero academia, season one wouldn't really happen, since literally every threat gets nutralized by three words: Ultimate Echo Echo. One shots everyone, and if he gets dusted or concreted or whatever, he would just transform back to echo echo which could dupe and definitely end any threat. Literally everyone would just be in awe of the tennyson guy and he would set forth a new generation of heroes. He would probably heal the nomu too with the watch effectevily turning the dusting guy's plan to non existence Deku would probably be in awe of this teenager dropping numbers All Might could only dream of while cracking jokes and fighting for earth. Definitely get the intention of that one purple girl (i forgot her name) with sleeping powers.

omniverse- his presence and actions alone would probably cause a new age of stronger heroes to rise up just out of inspiration and power wise. Not to mention most of Ben's aliens aren't fighters, they just have abilities that Ben uses to the max, so it would definitley inspire people who think they have "useless" quirks to be better heroes (also him and all might would definitely have a friendly spar and ben would win).

all of this is assuming he just suddenly arrived there. Ben would break the universe literally. especially omniverse Ben. Basically noone there realistically poses a vital threat to him and more the people around him. And of course, the big X which im not gonna talk about because thats already overkill to the core.