This is an excellent idea. And a really empty space market-wise. Hurry, and you will corner it!
I would suggest the following (all in 1/72, because that's were most the kits are, but it's easy to scale them up to 1/35. Once you've done the masters, the scale is pretty easy to change.)
1. One sheet with numbers and nicknames for British Mk I, II, III and IV, and Whippet. Caters for the Airfix Mk I and Emhar Mk IV. (Plus a number of White Metal and Resin kits.)
2. One sheet with numbers, nicknames etc for the German A7V. Caters for the Emhar A7V. (Plus a lot of Resin and White Metal kits etc.)
3. One sheet with German markings used on captured tanks. Caters for the Emhar Mk IV. (Perhaps this one could be incorporated into sheet 1?)
4. One sheet with markings for French WW1 Tanks. Principly the FT-17, catering for the RPM kit, but also for the coming kits by HäT and Mirage - plus the usual array of White Metal and Resin kits etc. But also some intended for the S:t Chamond and Schneider CA.1 - which you also can find in other materials than plastic.
5. One sheet for Russian Armoured Cars. Both Imperial Army and Russian Civil War. There is no plastic kit out on this subject YET, except one Russo-Balt, but there are LOTS of resin and white metal kits!
That's my suggestion. I'll be glad to help you with any info you need. I have much material. You can also take a peek at my site: http://www.landships.freeservers.com
This is a great idea. It is pristine territory marketwise, at the same time that decals would surely give yet another boost to a growing segment of the hobby!
All the best