Even at their best, expansions or DLC are inferior to continuous development or just making a straight sequel.
Look at any Paradox Game, or AI War - the game ends up cluttered with disjointed systems that work in complete isolation from one another. Menu is heaped on menu, resource on resource, because they can only be balanced by keeping them apart from one another, since you cannot be sure which ones players buy or leave out.
And if the slipperly slope is slid down, necessary maintenance work and rebalancing that should have been in the base game is then moved into DLC, and the vanilla experience degrades over time. And even if DLC is limited to adding content instead of adding systems, what does that say about the content? Is it meaningless padding, re-hashing preexisting content? Or is it actually game-changing? If the latter, why is it not in the base game?
You can make an argument for DLC as massive sidegrades that completely change the nature of the game in ways that not all players may desire and should thus be optional, but then you essentially split the game and the developer's attention in two. Not a good deal, long-term.
A single-seat studio like Alex should not split its attention thus. I strongly recommend that the game be kept in one single piece - and, if it seems sound in terms of marketing, with a sequel in the far future.