Do you have any ideas / suggestions what kind of kingdom devout, practicing Born Again Christians can inherit, that completely reject Joseph Smith and classify the LDS church as a cult?
Unfortunately, the answer to your question is unknowable. All of God’s children will receive whatever He deems is an appropriate opportunity to accept the fullness of the gospel of Jesus Christ. Those who accept the gospel when this opportunity is provided can expect to receive all blessings as if they had lived their entire lives adherent to it. This is exemplified by parts the parable of the laborers in the vineyard, as recorded by Matthew:
For the kingdom of heaven is like unto a man that is an householder, which went out early in the morning to hire labourers into his vineyard.
And when he had agreed with the labourers for a penny a day, he sent them into his vineyard.
. . .
And about the eleventh hour he went out, and found others standing idle, and saith unto them, Why stand ye here all the day idle?
They say unto him, Because no man hath hired us. He saith unto them, Go ye also into the vineyard; and whatsoever is right, that shall ye receive.
So when even was come, the lord of the vineyard saith unto his steward, Call the labourers, and give them their hire, beginning from the last unto the first.
And when they came that were hired about the eleventh hour, they received every man a penny. (Matt 20: 1-2, 6-9)
Everyone who accepts the gospel according to Christs terms, can expect to inherit a fullness of His recompense. However, even those who may appear to outright reject the gospel might expect to still receive glory, although of a lesser sort. Through modern revelation with which God has blessed us, we can understand some of the types of circumstances that would cause someone to inherit a lesser glory. One such revelation is found in the Doctrine & Covenants, Section 76, which states, in part:
And again, we saw the terrestrial world, and behold and lo, these are they who are of the terrestrial, whose glory differs from that of the church of the Firstborn who have received the fulness of the Father, even as that of the moon differs from the sun in the firmament.
Behold, these are they who died without law;
And also they who are the spirits of men kept in prison, whom the Son visited, and preached the gospel unto them, that they might be judged according to men in the flesh;
Who received not the testimony of Jesus in the flesh, but afterwards received it.
These are they who are honorable men of the earth, who were blinded by the craftiness of men.
These are they who receive of his glory, but not of his fulness.
These are they who receive of the presence of the Son, but not of the fulness of the Father.
Wherefore, they are bodies terrestrial, and not bodies celestial, and differ in glory as the moon differs from the sun.
These are they who are not valiant in the testimony of Jesus; wherefore, they obtain not the crown over the kingdom of our God. (D&C 76: 71-79)
Even having the amazing insights afforded by modern revelation, although we can understand the types of situations that will result in someone inheriting a lesser glory or kingdom, we remain unequipped to be able to pass judgement on any any specific individual’s situation, as it is the Lord who knows each of us, and our hearts, and will be the one to pass final judgement. I encourage you to study Doctrine & Covenants, Section 76 for yourself, as it can serve as an excellent introspective tool. Ultimately, when Judgement Day comes, each of us will know, understand, and fully accept the Lord’s Judgement, and will recognize it as both just and true.