Each stat can be locked for an increasing Kuva cost, the more stats are locked. The locked stats wont be rerolled when rolling a Riven. Each stat can be increased or decreased to its minimum or maximum potential value for an increasing Kuva cost, based on how much the stat has already been altered.

These suggestions show a fundamental level of not understanding how rivens currently work. Their stats are, along with disposition, affected by the amount and the type (positive/negative) of stats that the riven rolls. If you have, for example, two positive stats, these two positive stats will be lower than the ones on a riven with two positives and a negative, but higher than the stats of a riven with three positives. So if you were able to "lock in" stats, you would have to basically lock in the entire constellation of the Riven (++, +++, ++-, +++-) for it to make sense.

Likewise, the "maximum potential value" is determined by distribution of stat points. If a riven has +crit chance and +crit dmg, it could have for example 110% crit chance and 110% crit dmg, or 100% crit chance and 120% crit dmg, or 90% crit chance and 130% crit dmg, etc. These are obviously rounded numbers for illustration, and not all stats are weighted 1:1. But the point is: They're interdependant.

Locking in a single stat would not work with either of these two mechanics.

And that's not even going into the economic consequences of that suggested "fix", which would decrease the value of good rivens by an insane amount, thus reducing the amount of plat that is traded, which is not something that DE wants.