I am running my groups 2nd 5e campaign. The main issue that my players have is that they do not feel that their characters gain power from one level to the next. They like the system overall, though. They do not see much difference between say a 5th level character and an 8th or 10th character. They would like some more some sort of bonuses or maybe adjust the proficiency bonus a bit higher maybe +8.
Any suggestions?
The idea, of course, is that you are getting better in the sense of making more attacks, having more hps, casting more & higher level spells, etc.
You could re-inject a little 'bigger numbers' feel by adding a +1 to proficiency here or there at levels that seem appropriate to you. When you run monsters of that CR or higher, just boost their ACs and saves by the same amount. The net effect will be that PCs are more effective against lower level creatures, making them appear to have gotten 'better,' but still face the same challenge against same-level monsters - and face a greater challenge from higher-level ones, giving them the sense that they still have 'room to grow.'
Of course, the more such bonuses you add, the more you'll un-do the effects of bounded accuracy, lower level monsters will eventually become pointless, higher level ones untouchable, if you were to, say, net double proficiency at higher levels.
Currently proficiency bonus increments at 5,9,13, & 17. You could change that to 4, 8, 12, 16, & 20. That's up to +7 prof. Make it 4, 7, 10, 13, 16, & 19 and that will take it up to +8, which was an idea you'd considered - it'd barely be perceptible, at 13th, PC's would be a net +1 vs lower-level monsters, and a net -1 vs 16th+ ones, relative to the normal progression. To be really noticeable, you'd have to and proficiency every-other level, +1 at each even numbered level, for instance, would double the maximium proficiency bonus from +6 at 17th to +12 at 20th. If you added that extra +6 over 20 levels to the ACs & saves (and save DCs) of monsters, PCs would start to notice that they 'couldn't miss' fractional CR monsters like goblins starting somewhere in double-digit levels. 1st level PCs would, similarly, be unable to hit very high level monsters. While neither of those conditions would be likely to make a huge difference to the outcome of battles, they might affect the feel.
Note, BTW, that PC AC doesn't generally increase with proficiency, so I didn't recommend boosting monster attack rolls, those low-level monsters will still hit the PCs about like always.
Also note that, since saves sometimes get proficiency & sometimes don't, while save DCs always add proficiency, that this variant would make it increasingly hard for PCs to make their 'bad' saves as they level. Likewise, the gap between proficient and non-proficient would widen, and Expertise would get pretty crazy. So you might want to think twice about using this proficiency bonus with skills.
Another thing you could try is different proficiency bonus progressions for different classes doing different things.
For instance:
'Good' proficiency: +2 at 1st, +1/4 level thereafter. (Most characters, for most things - skills attack bonus, good saves, save DCs etc).
'Poor' proficiency: +0 @ 1st, +1/4 level thereafter. (Bad saves, wizard attack bonus with weapons, EK & CT save DCs)
'Expert' proficiency: +2 at 1st, +1/2 levels thereafter. (Champion, Battlemaster, & Berserker attack rolls; Rogue & Bard 'Expertise' skills)
