HELL NO !!!
Don't worry, not only you will have this, everyone will!
Yes, if you want to try other build... well.. that's a problem.
But if they want to try other build, they will have the same problem!
Total: everyone will have problems in making other builds.
So you need to find a solution to this, may be something like reseting character without losing XP, but then everyone will make HtH/Melee, lvl cap him and only then create evil powerbuild. So there must be an account system with few characters and some achievements like killed X mobs with EW/SG/BG/Melee/HtH/Throwing, then every character with atleast one of those skills tagged will exp faster. Different achievements with quests, every other character in account completing the same quest will get XP boost, exping one character to max lvl will allow to have Swift Learner perk (+5% to xp) at 1st level for every new created character, having 2 max lvled characters on account will make next created character have x2 Swift Learner perks at creation.
But then again, XP diminishing should be reworked, because little scorpions who giving ~60 XP are counted the same in kills as big scorpions who giving 110 XP or even mutated scorpions who giving don't know how much XP, making players to ignore those little ones and kill only big/mutated, to get as much XP as possible and it looks really strange. I can remember those days where you had to kill 12 mutated molerats, 12 big scorpions, 12 mutated mantises etc. to get big XP boost at begin, and if you meet pig rats or small scorpions or simple mantises, you need to ignore them, and in scorpions encounter you need to kill only big one, and run away from small ones.
Edit:
Do you think that people whch sitting in front of computer whole day will have problem ?
Now everyone is on same level ....
Why you want make more pain in ass ? Its test, we want testing new build and etc, after wipe there will be many ways to have different builds. Its TESTING, you knew it ? If you ask me if i want test my patience when i must spend biliion of ammo to earn 200xp for floater then i say no
And of couse everything after line:
"Total: everyone will have problems in making other builds."
you forgot to read... I already answered on your post.
Edit#2:
Make Hinkley give you XP to max lvl (with all perks, so you'll get next level after pressing "OK" in character sheet, level by level).
But this character can't leave Hinkley! He'll have "Combatant tatoo" on his forehead, everyone outside Hinklet will burst him with words "Cheater! How did you left Hinkley?" and then he'll be respawned in Hinkley, if by some reason he was teleported to world map or something. Also he'll get random encounters with Hinkley slavers who will hunt down escaped combatants!
So you can test your builds there!