Why is it sem not tady for Here?
Tady is used for location, while sem is used for direction (basically "to here").
Could "přijdeš" be accepted here?
"přijdeš" turns it into the future tense: "Will you come here?".
No, not really. Přijdeš implies future.
Jdeš sem? = Are you coming here? / Are you on your way here?
Přijdeš sem? = Will you come here?