Based upon the original post's questions:
I guess I'd have to ask, which game? I figure you mean D&D, but D&D hasn't been the same game throughout its' history. I think most rules would need to be changed within the current version.
However, I believe
4E did go back to a flatter "curve" reminescent of earlier pre-d20 versions of D&D. The progression slope is lower than
3E. This could work, but not for 30 levels. It might work if all new PCs started at the starting level for a tier (the lowest level of the tier). ...and no PCs in the party spread across two tiers. Essentially the whole party retires and ends the game after one tier and then begins the Paragon or Epic game.
How many hit points per level? At least 1 HP for starting level PCs. At least 10 for 10th level. Hit Die works as well for greater variation and enjoyment for the players. 20th level fighters? Way out of bounds unless you half again the progression slope. And giving half a hit point per level is a little sketchy
IMO.
Suggestions? I suggest every PC have a base chance for failure and a base chance for success regardless of their level for ALL class related challenges. (Ignoring gear, assistance, negotiation, etc.) fighters have a 5% chance of success at 1st level against a 10th level monster and 50% against a 1st level monster. The success chance becomes completely inverted upon reaching 10th level: 95% chance of success against 1st level monsters, 50% against 10th level monsters.
D&D does (or did) this full progression across one die, one die roll being the simplest. Think THAC0 progression. Do you need an 11 to defeat the enemy or a 20? A 2 or an 11? Are they the easiest to defeat or the hardest? As always the difficulty of the game depends upon one's level bonuses earned so far.
Of course going up against a 10th level challenge makes little sense even if it is possible at 1st. The key factor for why players wouldn't normally take such foolishly low odds for success is failure would likely mean they would begin the game again with zero points and level 1 bonuses. As one goes up in level the amount of effort lost upon failing to beat a challenge goes up too. So players will almost certainly become more cautious at higher levels. A good thing, if the point is to stay alive and retain one's XP total. However this desire to stay alive is mitigated by the player's desire to actually gain more XP and access to even higher bonuses, so I think it works out well in the end.
In terms of XP rewards, I'd award players for roleplaying well enough to overcome the challenges with a similarly progressive XP chart. Those playing 1st level PCs receive very big ##'s for overcoming a 10th level monster, those playing 10th level PCs receive very small ##'s for overcoming a 1st level monster and those who overcame a level equivalent monster receive the level standard. But as levels go up, so must the standard point reward for any given level. Therefore we get a logarithmic XP reward progression for monsters / challenges. And because of this a logarithmic progression is best for level requirements too. This also helps those with lower level PCs catch up as, with XP charts which double each level, the highest level PC is only going to advance a single level in the same amount of time it takes for everyone else to attain that high PC's level when they began play. (I hope that makes sense) Of course, this is all depends upon individual play results too.
Our group has been playing with every PC starting at 1st level for over 5 years now and the game has worked well throughout. We've had young to old, very experienced to complete newbie players. No one has really had a problem with it either as so much of the game is about personal success even in the midst of working together as a team.
I find it's best to remember that players are not in competition with each other, but against the game. Just because someone has more points or a higher level PC does not mean they are beating you. I've seen 1st level 1 hp PCs do things my 5th level PC never could as their players outwit high level monsters (not to mention my PC on occasion - a "monster" to the other PCs technically anyways).