The site seems to already try to track playtime to beat but i would suggest to expand a bit more on that since games are so diverse.

I would suggest for the site to add "Campaign information" to each Main type game (on bundles you see each bundled game having its own). Each game will have a list of types of gameplay they offer, the ones i came up with are (each game should have at least one):

Main Campaign: for games that have one big campaign ending usually at the credits (Ex: Undertale, Portals, Super Mario World, etc.). Games with multiple routes or endings should still count as one Main campaign.

Aftergame Campaign: this one is only unlocked after you finish the game and it only applies for games that do have this feature, it could be new levels, an extra story, new game +. Games should include a non marginal amount of new aftergame content to merit this.

Side Campaign (s): for games with a main campaign that include an optional, outside of main, not required to see the credits campaign. Dlcs, Mods, Seasons, Standalone Expansions and Expansions could be side stories of their main games if they apply as such. (Ex: Lisa the Joyful (DLC of Lisa the painful), New game + in various games, Celeste Farewell (DLC of celeste), Bowsers fury (Super Mario 3d world + bowsers fury). Games where its main gameplay is not a main Campaign (Party games, fighting games, etc.) but do have a side story mode do also apply for this (Ex: Subspace Emissary in Smash bros Brawl).

Continuous Campaign: this is a special category for all types of gameplay that do not intend to have a concrete ending, it would be party mode for party games, fighting mode for fighting game, point racking infinite games (like some arcade, phone or roguelike games), competitive multiplayer games (like some mobas, esports games), life sim games (like the sims, animal crossing), social games (like Habbo, second life, Kind words, etc), creative games (like Mario maker or flipnote studio), live service games (like MMos) and even an overlap of all of the above or multiple different modes. On these games progression may still be present in unlockable or achievements, but on their core they could be played endlessly until the player decides its over.

All games can have a maximum of one type of each (except for Side campaign, games can have multiple of those) on their Data. All games must have at least one type of Campaign and cant only be consistent of Side campaigns. After each corresponding campaign is added, users on igdb can rate their playtime to beat and 100% completion of each campaign. For the continuous campaign it works differently, playtime is measured instead of beat time and instead of 100% it measures the time to collect all achievements/unlockables.

With these changes people can more accurately judge the playtime of a game and it will also help nuance a little bit more on which type of game is being shown. With this information also backlog sites that use the IGDB API can better help their users track their own gameplay, what they need to complete and what kind of game they wish to play.