This is tough when you say "no new graphics". You could make it a skiller area by adding anvils, trees, rocks, etc, but there is no room for any of that unless the guild is expanded or had a basement added. You could add a passive exp bonus to all skills while inside the guild I guess?
I feel like the max guild is fine as it is - does it really need to be amazing? The max cape by itself is already a big reward with its stats and ability to store 3 skillcape perks. Free teleport right next to the bank/GE. The guild will still have a slightly cheaper boss portal. I think maxing is still a worthwhile achievement based on those.
I'd love to know what the jmods currently have for ideas. Maybe give us a better idea on what you're looking for with suggestions.