Unfortunately we can't determine the plant's ID at this stage -- it's too young, and as you noted, flowers can be key to narrowing-down and confirming ID. This plant currently resembles a number of unrelated species, like fleabane (Erigeron), marestail, lobelia, and campion (Silene). We're happy to take another look as it matures if you're willing to keep the plant for that long (as opposed to weeding it out).

Miri