I'm facing a similar issue with a game I'm developing. It would theoretically work better with PvP, but as a small indie developer, you simply can't count on having the player base needed to sustain multiplayer. My situation is slightly different, though, since I originally designed the game as a single-player roguelike, with multiplayer as a potential addition. So my approach is to focus on making the single-player experience good enough to stand on its own, then introduce multiplayer if there's sufficient interest. Bots can certainly help, but only if the game is simple enough that they can provide a meaningful challenge without requiring particularly sophisticated AI.