Well im glad this said a bit more than i thought, Achievements in the gaming world to me means them things they have on xbox... god i hate that, i think the system is great but so many games was ruined because they just said 'do this' 'do that' and get gamerscore... then people would rather compete for uping the number than playing for a laugh... then again i dont like gaming much (outside visual novels) so i suppose thats me
anyways glad you didnt say implement gamer score
Basically your idea sounds like a log book to me in which you tick the enteries of one by one...
take below, a simple image of a incomplete CG gallery in a game (yes start ein no aselia recently plays well here
notice its of course incomplete... but how doi know? well them blank windows tell me instantly
but the courner... notice 36% completion, that tells me i have about a third of the CG's for Lesteena, so it tells me i got more for Lesteena to do (be supprised if i didnt lol)
Anyways point is i know i have more to do so its practically a given, though it doesnt directly tell me i might need to have her hate me or love me for a CG it doesnt change i know theres something to do but now lets change it up for a better example.
This is a more effceient one.
Notice here it tells me what i done?
i can see i have beaten all the endings...
i can see somewhere in the game im missing a single scene
i can see i still have little over 50 paths i havent taken
and i can see i got all the screen savers...
(and weirdly enough i can see im 112%/114% done... might aswell be 98% lol)
anyways thats my overview i know what i done but it dont show me what im missing then, but it does, i move to page 2:
Here i can see im not missing a ending, honestly for my example i wish i was but say on this list, if assuming i didnt have 'Sara bad ending' i can instantly guess what it is after playing for a little while, by guessing looking at my unlocks i can be 80% certain her missing one is a 'bad ending' due to other characters all featuring a bad ending aswell.
Now remember the missing scene? righty so heres some random point in my scene log:
all are fine and done, but now look at the missing scene:
u ccan tell instantly its missing hense the '????'
this tells me something most likly i missed something between the last event and the first event
fortunatly it was placed effeciently...
(i actually now know where the scene is xD) but basically i can guess its in two places:
note the 2 surrounding it:
'Kid/Sara Future' <- i know this is the very very end of Sara's story, infact its her epilouge, so realisticly chances of it being in her epolouge after reciving one there is extreemely low, probberly less than 10% if not less than 5.
so i look at point 2:
'Coco Who am i, who are you?' <- this tells me its before here, well i know this is the start of the very game so i instantly know theres a good 80% chance it happens around here
Now this doesnt feel like it realtes alot right? maybe it doesnt, what im trying to show is a normal CG gallery offers you knowledge you havent done something, im one who loves exploring what that something is, i would upload my cg from Metero downhill type R for proof that on there i have done all main stories but missing a few cg's and havent a clue how to get em but i like exploring, but as said some might like it to tell them so your system works if maybe when i hover over a blank frame it says:
'unlock kyouko good end to get this' or i just have a screen like the log in the examples but instead blocking the title saying say 'Sora good ending' it might say 'Sora bad ending beat' and have something like 'beat sora good ending to unlock this Achievement' or whatever
basically what im saying is your idea is great for some, some would priase it well, i myself enjoy finding whats missing alone, so though i might know i have Cg's for Aselia i might be missing one which i dont know what it is, this is where your system works for some, but for me i rather play Aselia till i find it, if i dont find it i know its gotten via someone else, its a longer process but i find it more enjoyable as you get the full most gameplay.
But what im saying it you could do your system quite well on middle ground as a log book as i put with the examples, they could either tell you how to get the missing scene or they could just leave it to your own logic and my exmaple does.
this is of course my view, your idea is a validly decent one though ^^