1. Tanaka(Tanaka-kun is always Listless) + Isekai + System
2. Tokyo Ghoul + MC(could be old [Ken Kanaki], could be new) has all Kagunes(origin/genesis ghoul as the race maybe) + system(maybe, at least a status sheet)
3. Plunder/devourer/copy + any series [we need more of these based series]
3a. Plunder + Tokyo Ghoul (maybe a kagune that plunders others / maybe evolving by absorbing)
3b. Plunder + Cultivation(maybe a plunder body)
3c. Plunder + MHA
3d. Plunder + Fairy Tail
3e. Plunder + Multiverse
3f. Plunder + Infamous (Basically an Infamous Second Son since the mc collected 4 of the conduit powers but beyond since it only went into 4 <Neon, Video, Smoke, Concrete>)
3g. Charlotte + any series
3h. Code Vein(the protagonist) + any series
3bonus. Infamous Second Son + any series(maybe in the form of a gamer system or something maybe the power in the form of where he gets the [ Conduit ] skill where as he levels it up he unlocks the different branches of conduit powers)
4. Hollow Knight + a series [someone already did this for MHA tho abandoned or so] p.s. you can find most likely any book I recommend in my reading lists
5. anime soul [watching anime allowed your soul to connect to others a certain Nani Da F*ck novel did something like this] + a series or create your own universe
6. Upgrade system + Legacies(fusing/upgrading them) [a certain douluo dalou upgrade system fanfiction did this]
7. Date A Live + another series [a certain Daoist_Zero did this and more]
8. PotK <Phantom of the Kill>(I haven't seen one that used this world in a fanfiction yet)
9. Tales of the Ray's (same as above)
10. Shield Hero(maybe not the hero but the legendary weapons) + another series/multiverse (There are probably a few of these fanfictions but I cannot find them)
10a. Shield Hero + gun legendary weapon
10b. Shield Hero + weapon masters legendary weapon/all legendary weapons <these 2 would be interesting>
10ba. Remember in DXD when the mc Issei absorbed an orb of Vali's sacred gear what if you did that with the 4 Legendary weapons that would be real interesting
11. Multiverse + Gourmet Cells(Toriko)
12. I Have a Divine Tree In My Heart + Multiverse
13. Metal Goddess Soldiers + transmigration + system <maybe> + harem + 3 clones <ex. Yamato, the sword of seperation> (for each fleet, have each clone be a different part of the original <so therefore at some point they can merge and still look at least some what the same if not the same ex. main body can have a tri colored hair and when he separates himself into the clones it is of a single color for each one> , maybe more)
14. Rewrite (anime) + Other series <reincarnate the main character cast / group into another world or something i myself would request it be harem alright>
14a. Sealed memories Tennouji wakes these back up
15. Rewrite alternate (Kotarou merges with all his alternate route(timeline) selves plus his future route timeline self) this creates a new route one that is the best
16. Fate (or any other universe/series really) + Characters from other series
16a. Fate (or any) + Shu Ouma (Guity Crown)
16b. Fate (or any) + Minato Arisato/Makoto Yuki (P3)
17. Bloodline Gacha or really any Gacha
18. Persona 3 + any series
19. D Gray Man + any series
20. Music + Any series
20a. Music Man Skill (A skill denoting that you're very existence would make people hear Music for example Doomguy from Doom)(maybe a sorta reality marble skill where he summons a concert or something)
20b. Musical MC (you know the normal way through concerts and whatnot)
21. Incarnation system (what I'm thinking for this line is that the system gachas incarnations awakening/making you an incarnation of that thing I recommend if you're gonna go down this path, to read Bonds and Magic KTM Project and Why Did You Summon Me? and take inspiration from these great books) + any series
22. Gacha Essence(CYOA?) + any series (someone has already done this on this website just look up gacha/gacha essence and there it is)
23. Warframe + any series <preferably with all weapons and warframes>
24. Merlin + any series <within reason obviously>
Check my reading list for either recommendations, references, and/or books that should be continued
Any ideas you come up with comment here