In Q:What does memento mori actually mean? there does not appear to be a natural conclusion. Apposite contributions appeared as comments but were not developed. Perhaps it was believed that the Q. had been answered. There still seems to be more to discuss.
The well-established translations: "Remember you must die" & "Remember you are mortal": firstly, the original Latin does not include "you" and/or a second-person singular verb, the translations do include "you", which may explain how confusion arises. Secondly, these two are essentially the same. Mortality is a susceptibility to death. If "you are mortal" then "you must die", by definition. Pavel V and Cerberus gave the literal translation "Remember to die" (In case we forget?) which, for them, includes an implied obligation: "Remember you must die" (sounds familiar) a veritable cascade of congruence possibly pointing to the correct answer.
Implied obligation: an English equivalent could be: "Remember to wash the pots." then "You'd better wash the pots." therefore "You must wash the pots." As geomars indicated there is an implied threat, here, as well as an obligation.
To introduce obligation, without threat, the gerundive (of an intransitive verb, used impersonally): nunc est moriendum = now one must die; or, mox erit moriendum = soon one must die; continuing (thanks to brianpck) "moriendum est omnibus" = "all must die". Alternatively, "tu es mortalis, morieris"; or, just "tu morieris" = you will die. It may be more prosaic; but, directly to the point; no equivocation; no implications.
A most interesting point was made by C.M. Weimer; "memento mori" may be treated as indirect. This solves the problem of the inclusion of pronoun "you", in the Latin. The man-in-the-chariot (interlocutor) advising our all-conquering hero:
"dicit memento te mori" = "he (interlocutor) says remember you are dying";
present tense accusative-infinitive construction.
Sanguine about death, Seneca, in his interpretation of Roman theology ("Epistle 1") counselled the wise use of time (hence life) as it passes, only to be scooped-up by Death, in His unrelenting approach to the individual:
"...qui intellegat se cotidie mori." = "...he, who may understand that every day he is dying.";
present tense, indirect statement with accusative reflexive pronoun.
The warning (indirect speech): "Remember you are dying." appears to be compliant with a Roman mindset.
Noteworthy, also, the second offer from Cerberus; "Remember Dying"; as a present participle; "Remember (I see you) dying."; as a gerund; "Remember the dying (of the light/ of the mortal.)"
Concluding: either "memento mori" is to be translated literally, with its implied obligation, giving: "Remember to die." = "Remember you must die."; or, as indirect speech: "(he says) remember you are dying."