We are moving to a D3-paragon-like skill tree which means experience will be constantly gained for a character benefit. I pumped thousands of hours into that game so I'm familiar with the system. The problem with incorporating that into MH is that MH sells XP potions. Those little buffs are a big part of how they make money. I have no problems funding this game; I have pumped a ton of money into it. But...
If XP is constantly gained for a constant character benefit then those who constantly buy XP buffs will become the dominant characters with an honourable mention going to botters. Now I wouldn't normally care how someone else conducts their affairs except that I really enjoy leaderboard challenges. Those challenges - at least the top rankings - will be forever out of reach for those who do not pony-up along with those special rewards we all try to grind for. This is the definition of pay-to-win.
This will also be discouraging for new players. If you come to this game sometime down the road with these infinite levelling systems in place, you know you will never, ever, be competitive as you can never bridge the gap between the high ranking players and yourself. So all those top-content rewards are not for you. Why play then? I left D3 over that. MH has allowed whoever put in the time, research, and/or ingenuity to count themselves amongst the elite when it comes to challenges. I fear we are trading that away for something much more expensive and much less rewarding.