I suggest a skill cap, or at least making skills harder to obtain after a certain point. This is equivalent to reaching level 60 (or 70 now, I guess) in World of Warcraft - at that point, no more skill can be obtained.
If players maxed out at Grandmaster level, this would make the game more strategic - at the moment, the higher tiers are primarily concerned with botting around the clock in an attempt to win the "Skills Arms Race".
Limiting players to a maximum of, say, 15000xp in offence and defence would encourage players to work in teams, rather than setting up for 24-hour bot runs. This would of course be hampered by the fact that the in-game systems don't yet support advanced chat options (like private messages), so it might be an idea which develops more in PK 1.5/2.
Does anybody disagree, and think that it's acceptable that higher-tier players will always remain untouchable by virtue of endless upskilling?