Hmm... Tough call. Portal gems are your bread and butter, so you don't want to dissuade the hardcore players from using them liberally. I keep looking at other games with micro-payment structures (We Rule for iPhone, Runes of Magic on PC), and they typically allow people dropping the cash to get quite a leg-up over those using the default free functionality, within bounds.
Portalling (saving) for guild/rest/level-up does not feel like cheating to me, and adds another dimension to the game. Using portals to raise stats without limit by way of tomes/rings *does* feel like an exploit, and I think you're right to consider putting in some safeguards.
I think
both the use-once stat items and a stat-cap are the way to go, with use-once items being the major mechanic to limit exploitation (esp. STR and DEX) without dissuading portal use.
Thinking out loud here, there are 2 types of stats: Those that affect immediate encounter survivability (STR, DEX, CON), and those that allow you extend your dungeon time/number of encounters (INT, WIS).
STR and DEX directly affect damage output, regardless of personal level. CON affects HP, though not retroactively, but can have a *huge* effect if buffed in new characters. INT and WIS both retroactively affect the number and level of spells available, but not their damage or chance to be resisted... A level 7 Ranger casting FIEMINAMOR isn't going to help much against Titans. A level 7 ranger with 200 STR/DEX and 999 HP is a different story.
So my recommendation here is that items that raise STR, DEX and CON are use-once items that raise a stat by 2. That's 2 levels worth of save/reload resting and very desirable drop. Items that raise INT should maybe be 8 points (which you'd roughly get from Magi tome), giving Mages/Sages two spell rank increases and Rangers one. Probably the same mechanic for WIS, but I never worked out the exact stat vs spell numbers for PIE.
There are already mechanics to limit super-expliot of INT, WIS and CON. Spells have a cap on how many can be cast per spell level, and HP are limited to 999. Still, you don't want level 5's with 999 HP TORGAFIERing around at will. STR and DEX really need to be capped or those level 5's could be pwning the deepest areas of the dungeon.
The stat cap should be nearly unattainable through regular play to encourage people to keep leveling and seeking stat-buff items, but not so high as to completely eliminate all risk on level 10. Even if they've min/maxed every level up to end-game, dungeon level 10 is still challenging with stats in the 40's and 50's (ignoring the old dex bug).
Implementing a level-dependent stat-cap kills a few birds with one stone. Even fresh characters 'twinked' by an end-game party couldn't be too overpowered, but a high stat cap for level 50's gives them a reason to keep playing.
The highest natural ability scores possible after a guild are I think:
- 48 STR (38base + 10guild), since Ogres can't naturally access guilds with +10 max to STR.
- 42 INT
- 42 WIS
- 42 LDR
- (43?) CON (38 base Ogre + 5 guild?)
- 42 Dex
If these are in the right ballpark, why not put in a stat cap of 48 + personal level with a max of 99? That way, you can still enjoy that exceptional new character you just spent 2 hours rolling (and a dozen gems guilding), max their stat gain at every level through gems/reload, but not overpower dungeon levels before your time with 200 STR/DEX. The use-once stat buffers will keep STR/DEX/CON in check, and even with several Magi drops, a level 40 Mage with 98 INT will *just* be hitting spell cap, which feels about right, considering it will still take 2-3 FIEMINAMORS at that level to down the toughest monsters.
TL/DR: Rolling stat cap of 48+ personal level, hard cap 99. All stat increase items are use-once with +2 for STR/DEX/CON/LDR and +8 for INT/WIS.There. That'll teach you to ask for my opinion :p