A bit on a tangent here, I know most gamers have their preferences towards game genres, so what different genre combinations would possibly hold as much of the market as possible? (Visually, which genre combinations would have the center portion of a Venn diagram largest?) What would you think about an action/adventure and town building hybrid game in terms of market?

That's an interesting and challenging question. I'd approach it by first looking at game types that are so ubiquitous we hardly even notice when they are blended into other games, and ones everyone has played. Games which the player already knows more-or-less how to play have a lower barrier to entry than really unfamiliar ones.

For example, "shopping" - A huge percentage of the games in existence contain some sort of money system, where the player receives income either as a direct reward, as a time-based or level-based stipend, or randomly. The player will generally have enough money to buy something but not everything. The player's choice of what to buy may be strategic if it affects game play, or may be satisfying in a different way if the choice is mainly aesthetic, such as being able to purchase various character customizations.

Another super-common gameplay element is "scheduled resource allocation". What that means is that the player makes a choice or set of choices about what will be automatically developed or invested in while they are doing something else. Some examples: owning only one flower pot and deciding which seed to plant in it, allocating your character's offline time in an MMO, deciding which pet or mount to equip because the equipped one passively gains a share of earned XP, or the rather famous materia-equippage system in FF7 where spell-gems equipped to weapon slots gained an XP share, weapons had varying numbers of slots which gave varying shares of XP, if you maxed out a gem it produced a baby one, and if you maxed out one of every color you could combine them into a big gem which could be equipped more efficiently.

Some other extremely common gameplay elements or activities players will already be familiar with from real life:
- Selling
- Card games, both solitaire and multiplayer games like poker, trick-taking games, shedding games, etc. Card-battling games are not going to be familiar to as wide of an audience but a large percentage of video gamers will have some familiarity with them.
- Non-card turn-based solitaires ranging from minesweeper, maze-navigation, and leapfrog pegs/marbles to adventure game puzzles.
- Real-time solitaires such as tetris, frozen-bubble, match-3
- Sim real-life activities such as gardening, fishing, cooking, and interacting with pets
- Efficiency games, these may include shopping as mentioned above but also include worker placement and task queuing.

On a different note, I think people in general react well to a game where the player can move, at their own choice or at a steady alternation, between a challenging activity (either strategy or speed and adrenaline, the player faces a constant moderate danger of losing or dying) and a relaxed sandboxy activity where there is no danger and instead there is room for creative customization or reading/hearing/seeing some story, or other slow-paced activities.