When I took the Extra exam, as with all other upgrades, I had the choice of retaining my current callsign or getting a new systematic callsign. I choose to keep KD0UDM. If I chose a new systematic call sign, I'd have to consider the current line up of the next block of call signs in Group A:
AD0SA-Z except AD0SB and AD0SC (vanity calls)
AD0TA-Z except AD0TD and AD0TY (vanity calls)
AD0UA-Z except AD0UG (vanity call)
AD0VA-Z except AD0VK and AD0VW (vanity calls)
AD0WA-Z except AD0WB and AD0WX (vanity calls)
There won't be a problem with Group A calls in Region 10 for some time since there is a total of 4,215 callsigns left. This is not the case in Region 4, where they ran out of Group A calls on 12/8/12 with AK4ZZ and now up it's to KW4QB in Group B as of this date.
I could always try for a Vanity callsign and fight for a 2x1 or 1x2 callsign. I don't think it's worth it - I could end up lousy callsign like W0OF or WO0F. Forget it. There's also the possibility of using my initials and have operators across the UK fall out of their chairs laughing when they hear me calling
CQ UK CQ UK CQ UK DE KB0NK K
And no, I don't want AD0SA (SturmAbteilung), AB0SD (SicherheitsDienst), AB0SX (see KB0NK, ante), AB0SV (the first two initials of an ex-girlfriend). Call me superstitious.