Well, I'd want something with both replayability and variety. So basically, 4X and sandbox games. The only problem is that I don't like those one bit. I'd sooner spend a year playing Tetris than "play" Minecraft for half an hour.
If playing online was an option, I'd go with a Quake-like shooter. Probably OpenArena, because I don't know if there are still servers for Q3A.
But since it isn't, the only thing I'm certain about is some roguelike - most likely TOME4. Finishing it with all the different classes and subvarieties would take a long while. And it's moddable, so I could entertain myself that way as well. In fact, I think programming would be my main "game" - judging from some recent indie RPGs, three years is nothing :)
A fighting game might work, if I could play with other people
and I knew these other people weren't the sort to shank me because they got pissed off. So probably not.
Something with insane griding potential might also work. Disgaea comes to mind. On that note, here's the top 20 longest games according to howlongtobeat.com:
1 Clicker Heroes 392 Hours
2 Kerbal Space Program 335 Hours
3 Monster Hunter Freedom Unite 251 Hours
4 RollerCoaster Tycoon 2 175½ Hours
5 Animal Crossing 171½ Hours
6 Rocksmith 2014 164 Hours
7 MAG 162½ Hours
8 Animal Crossing: New Leaf 160½ Hours
9 The Legend of Zelda Collector's Edition 160 Hours
10 Might and Magic: Heroes VI 156 Hours
11 Monster Hunter 3 Ultimate 152 Hours
12 The Elder Scrolls V: Skyrim - Legendary Edition 151½ Hours
13 Gran Turismo 4 139 Hours
14 Monster Hunter Tri 135½ Hours
15 Dragon Warrior VII 135 Hours
16 The Elder Scrolls III: Morrowind 134 Hours
17 Mount & Blade: Warband 133½ Hours
18 Animal Crossing: Wild World 133 Hours
19 Frequency 130 Hours
20 Crusader Kings II 129 Hours
Well, there's stuff wrong with that list: #1 is a clicker "game", and only two users, so let's chuck that out. #6 doesn't seem to be a game, likewise for #19. And #7 is online multiplayer only, so it's out as well. So we need four more titles:
21 Monster Hunter 4 Ultimate 129 Hours
22 White Knight Chronicles II 128 Hours
23 Animal Crossing: City Folk 127½ Hours
24 Record of Agarest War 120 Hours
Egads, more Monster Hunter and Animal Crossing.
Same idea, but from gamefaqs.com, min 50 votes, adjusted for duplicates (same game, different platform) and MMORPGs:
1 3DS Monster Hunter 4 Ultimate 4.65 4.30 80.00+
8 3DS Monster Hunter 3 Ultimate 4.45 3.86 80.00+
9 WIIU Xenoblade Chronicles X 4.44 3.28 80.00+
10 PS4 Disgaea 5: Alliance of Vengeance 4.54 3.05 80.00+
12 PC The Witcher 3: Wild Hunt 4.52 3.16 80.00+
14 WII Xenoblade Chronicles 4.57 3.15 80.00+
17 PS4 Final Fantasy X / X-2 HD Remaster 4.45 3.04 80.00+
18 VITA Disgaea 4: A Promise Revisited 4.48 3.21 80.00+
20 PSP Monster Hunter Freedom Unite 4.49 4.01 80.00+
21 XBOX The Elder Scrolls III: Morrowind - Game of the Year Edition 4.36 3.31 80.00+
25 3DS Animal Crossing: New Leaf 4.50 2.54 80.00+
26 VITA Sword Art Online: Hollow Fragment 4.10 3.15 80.00+
28 PC Europa Universalis IV 4.40 3.88 80.00+
31 PS Dragon Warrior VII 4.08 3.44 80.00+
33 XONE Dragon Age: Inquisition 4.28 3.29 80.00+
34 VITA Destiny of Spirits 3.51 2.96 80.00+
41 XONE Fallout 4 4.48 3.18 80.00+
42 PC Crusader Kings II 4.30 3.72 80.00+
43 3DS Persona Q: Shadow of the Labyrinth 4.33 3.34 80.00+
44 PS3 Kingdom Hearts HD 2.5 ReMIX 4.65 3.18 80.00+
Phew, that should be twenty. Annoying. Since they're all just "80.00+", I'm not sure how they differentiated between them, which makes this one somewhat less useful. But it's mostly the same games. Monster Hunter ad nauseam.