You need to start by deciding what it doesn't do at the minute you would like it to do, and what colour you want it.
You are going through the same dilema I did a few months back. This year is my 15th anniversary, and I decided a modernisation was best for my Tazmon.
Current build includes:
Judy SL forks with Englund Air Carts
Mavic 517 Rims with Goldtec Hubs
Avid SD7 VBrakes and Levers
Middleburn RS8 cranks and rings
USE Alien Post & Flite Saddle
M950 XTR Rear Mech
LX Front Mech
XT 8 Speed Shifters
No Name riser bars and stem
I decided that I wanted to sympathetically modernise the brakes, suspension and tyres, but wanted it to still look like a 15 year old bike.
So sitting in the shed I have:
Sid RLT Ti forks
Avid BB7 Discs
Hope Pro II Evo hubs with Stans Crest Rims & Rocket Ron tyres (ready for the leap to tubeless)
Easton EA70 Risers
Still need to source an 8/9 speed XTR front mech that will fit and a shiny new stem (purely a vanity thing).
I will probably be respraying it in a different colour to its original yellow (probably a light blue) but will be fitting period correct decals. Because of the colour I doubt I will get away with too much bling, but Gold anodised might look ok.
Because I am not trying to make it look like a brand new model I have gone with cable discs so I can keep my original levers and use the cable guides properly, I won't be upgrading to 9/10 speed because I don't see an advantage and all my bits work fine.
It will hopefully end up being exactly what I need, which is the same sweet handling I have gotten used to for 15 years, but with better front suspension and brakes I don't mind using in the mud.
In order of importance, I would:
Replace anything tatty, the tyres/grips/cables/chain.
Stick a new fork in it, along with new wheels and brakes.
Consider a new rear shock (what do you have at the minute?).
Lastly respray, unless that is really important to you. Takes a while to get done, can be expensive, and obviously everything needs stripping.
As you say it fits so well, there is no point changing the bars and stem, unless they look tired or you fancy a new riding position. Likewise with the saddle and post, no point changing really unless they look knackered.
Unless you want 10 speed, barring condition, there is no reason to change the shifters and mechs.
Keep us posted.